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[!NOTE|label:references:]

stylus

[!TIP|label:references:]
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- gerrit comments:

tricky

:not(a[class*="btn"]) == :is(a:not([class*="btn"])

i.e.:

h2 > :not(a[class*="btn"]),
h2 > :is(a:not([class*="btn"]),

using bas64 string for svg

original

background-image: url("https://path/to/icon.svg")

using data:image/svg+xml;base64

generate bas64 encoded string
```
$ cat icon.svg | base64
```

using bas64 string in css

background-image: url(background-image: url("data:image/svg+xml;base64,......")

system font stack

[!TIP|label:references:]
-apple-system targets San Francisco in Safari (on Mac OS X and iOS), and it targets Neue Helvetica and Lucida Grande on older versions of Mac OS X. It properly selects between San Francisco Text and San Francisco Display depending on the text’s size.
system-ui represents the default UI font on a given platform.
BlinkMacSystemFont is the equivalent for Chrome on Mac OS X.
Segoe UI targets Windows and Windows Phone.
Roboto targets Android and newer Chrome OS. It is deliberately listed after Segoe UI so that if you’re an Android developer on Windows and have Roboto installed, Segoe UI will be used instead.

body {
  font-family: -apple-system,
               system-ui,
               BlinkMacSystemFont,
               "Segoe UI",
               Roboto,
               "Helvetica Neue",
               Arial,
               sans-serif
}

/* Wordpress 4.6 */
body {
   font-family: -apple-system,
                BlinkMacSystemFont,
                "Segoe UI",
                Roboto,
                Oxygen-Sans,
                Ubuntu,
                Cantarell,
                "Helvetica Neue",
                sans-serif;
}

/** github **/
body {
  font-family: -apple-system,
               BlinkMacSystemFont,
               "Segoe UI",
               Roboto,
               Helvetica,
               Arial,
               sans-serif,
               "Apple Color Emoji",  /* Emojis*/
               "Segoe UI Emoji",     /* Emojis*/
               "Segoe UI Symbol";    /* Emojis*/
}

explain
FONT
DEVICE TARGETED
-apple-system (San Francisco)
iOS Safari, macOS Safari, macOS Firefox
BlinkMacSystemFont (San Francisco)
macOS Chrome
Segoe UI
Windows
Roboto
Android, Chrome OS
Oxygen / Oxygen-Sans
KDE
Fira Sans
Firefox OS
Droid Sans
Older versions of Android
Ubuntu
Ubuntu
Cantarell
GNOME
Helvetica Neue
macOS versions < 10.11
Arial
Any
sans-serif
Any
comparison between the font stacks used by sites
WORDPRESS
MEDIUM
GHOST
GITHUB
-apple-system
-apple-system
-apple-system
-apple-system
BlinkMacSystemFont
BlinkMacSystemFont
BlinkMacSystemFont
BlinkMacSystemFont
"Segoe UI"
"Segoe UI"
"Segoe UI"
"Segoe UI"
Roboto
Roboto
Roboto
Roboto
Oxygen-Sans
Oxygen
Oxygen
Helvetica
Ubuntu
Ubuntu
Ubuntu
Arial
Cantarell
Cantarell
Cantarell
sans-serif
"Helvetica Neue"
Open Sans
"Fira Sans"
sans-serif
Helvetica Neue"
"Droid Sans"
sans-serif
"Helvetica Neue"
sans-serif

bootstrap

references:

using boostrap sytles

[!NOTE|style:callout]

bootstrap 4.x

<head>
<link href="https://getbootstrap.com/docs/4.0/assets/css/docs.min.css" rel="stylesheet" id="bootstrap-css">
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" rel="stylesheet" id="bootstrap-css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
</head>

<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <!-- Bootstrap CSS -->
    <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.2.2/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-EVSTQN3/azprG1Anm3QDgpJLIm9Nao0Yz1ztcQTwFspd3yD65VohhpuuCOmLASjC" crossorigin="anonymous">
</head>
<body>
    <!-- Bootstrap JS Bundle with Popper -->
    <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.0.2/dist/js/bootstrap.bundle.min.js" integrity="sha384-MrcW6ZMFYlzcLA8Nl+NtUVF0sA7MsXsP1UyJoMp4YLEuNSfAP+JcXn/tWtIaxVXM" crossorigin="anonymous"></script>
</body>

4.x

<!doctype html>
<html lang="en">
  <head>
    <!-- Required meta tags -->
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

    <!-- Bootstrap CSS -->
    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@4.0.0/dist/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">

    <title>Hello, world!</title>
  </head>
  <body>
    <h1>Hello, world!</h1>

    <!-- Optional JavaScript -->
    <!-- jQuery first, then Popper.js, then Bootstrap JS -->
    <script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
    <script src="https://cdn.jsdelivr.net/npm/popper.js@1.12.9/dist/umd/popper.min.js" integrity="sha384-ApNbgh9B+Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q" crossorigin="anonymous"></script>
    <script src="https://cdn.jsdelivr.net/npm/bootstrap@4.0.0/dist/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>
  </body>
</html>

5.x

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>original bootstrap 5.x alert</title>
    <!-- Bootstrap CSS -->
    <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.2/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-EVSTQN3/azprG1Anm3QDgpJLIm9Nao0Yz1ztcQTwFspd3yD65VohhpuuCOmLASjC" crossorigin="anonymous">
</head>
<body>
    <h1>Hello, world!</h1>
    <!-- Bootstrap JS Bundle with Popper -->
    <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.0.2/dist/js/bootstrap.bundle.min.js" integrity="sha384-MrcW6ZMFYlzcLA8Nl+NtUVF0sA7MsXsP1UyJoMp4YLEuNSfAP+JcXn/tWtIaxVXM" crossorigin="anonymous"></script>
</body>
</html>

alert

references:

bootstrap alert

bootstrap alert 4.x

.alert{ position:relative; padding:.75rem 1.25rem; margin-bottom:1rem; border:1px solid transparent; border-radius:.25rem }
.alert-heading{ color:inherit }
.alert-link{ font-weight:700 }

.alert-dismissible{ padding-right:4rem }
.alert-dismissible .close{ position:absolute; top:0; right:0; padding:.75rem 1.25rem; color:inherit }

.alert-primary{ color:#004085; background-color:#cce5ff; border-color:#b8daff }
.alert-primary hr{ border-top-color:#9fcdff }
.alert-primary .alert-link{ color:#002752 }

.alert-secondary{ color:#383d41; background-color:#e2e3e5; border-color:#d6d8db }
.alert-secondary hr{ border-top-color:#c8cbcf }
.alert-secondary .alert-link{ color:#202326 }

.alert-success{ color:#155724; background-color:#d4edda; border-color:#c3e6cb }
.alert-success hr{ border-top-color:#b1dfbb }
.alert-success .alert-link{ color:#0b2e13 }

.alert-info{ color:#0c5460; background-color:#d1ecf1; border-color:#bee5eb }
.alert-info hr{ border-top-color:#abdde5 }
.alert-info .alert-link{ color:#062c33 }

.alert-warning{ color:#856404; background-color:#fff3cd; border-color:#ffeeba }
.alert-warning hr{ border-top-color:#ffe8a1 }
.alert-warning .alert-link{ color:#533f03 }

.alert-danger{ color:#721c24; background-color:#f8d7da; border-color:#f5c6cb }
.alert-danger hr{ border-top-color:#f1b0b7 }
.alert-danger .alert-link{ color:#491217 }

.alert-light{ color:#818182; background-color:#fefefe; border-color:#fdfdfe }
.alert-light hr{ border-top-color:#ececf6 }
.alert-light .alert-link{ color:#686868 }

.alert-dark{ color:#1b1e21; background-color:#d6d8d9; border-color:#c6c8ca }
.alert-dark hr{ border-top-color:#b9bbbe }
.alert-dark .alert-link{ color:#040505 }

template

<div style="position:relative; padding:.75rem 1.25rem; margin-bottom:1rem; border:1px solid transparent; border-radius:.25rem;">
  <h4 style="color:inherit">X Alter !</h4>
  This is a x alert with <a href="#" style="">an example link</a>. Give it a click if you like.
</div>

bootstrap alert 5.x

.alert{ --bs-alert-bg:transparent;--bs-alert-padding-x:1rem;--bs-alert-padding-y:1rem;--bs-alert-margin-bottom:1rem;--bs-alert-color:inherit;--bs-alert-border-color:transparent;--bs-alert-border:1px solid var(--bs-alert-border-color);--bs-alert-border-radius:0.375rem;position:relative;padding:var(--bs-alert-padding-y) var(--bs-alert-padding-x);margin-bottom:var(--bs-alert-margin-bottom);color:var(--bs-alert-color);background-color:var(--bs-alert-bg);border:var(--bs-alert-border);border-radius:var(--bs-alert-border-radius) }
.alert-heading{ color:inherit }
.alert-link{ font-weight:700 }

.fade{ transition:opacity .15s linear }
.fade:not(.show){ opacity:0 }

.btn-close{ box-sizing:content-box;width:1em;height:1em;padding:.25em .25em;color:#000;background:transparent url("data:image/svg+xml,%3csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16' fill='%23000'%3e%3cpath d='M.293.293a1 1 0 0 1 1.414 0L8 6.586 14.293.293a1 1 0 1 1 1.414 1.414L9.414 8l6.293 6.293a1 1 0 0 1-1.414 1.414L8 9.414l-6.293 6.293a1 1 0 0 1-1.414-1.414L6.586 8 .293 1.707a1 1 0 0 1 0-1.414z'/%3e%3c/svg%3e") center/1em auto no-repeat;border:0;border-radius:.375rem;opacity:.5 }
.btn-close:hover{ color:#000;text-decoration:none;opacity:.75 }
.btn-close:focus{ outline:0;box-shadow:0 0 0 .25rem rgba(13,110,253,.25);opacity:1 }
.btn-close.disabled,.btn-close:disabled{ pointer-events:none;-webkit-user-select:none;-moz-user-select:none;user-select:none;opacity:.25 }
.btn-close-white{ filter:invert(1) grayscale(100%) brightness(200%) }

.alert-dismissible{ padding-right:3rem }
.alert-dismissible .btn-close{ position:absolute;top:0;right:0;z-index:2;padding:1.25rem 1rem }

.alert-primary{ --bs-alert-color:#084298;--bs-alert-bg:#cfe2ff;--bs-alert-border-color:#b6d4fe }
.alert-primary .alert-link{ color:#06357a }

.alert-secondary{ --bs-alert-color:#41464b;--bs-alert-bg:#e2e3e5;--bs-alert-border-color:#d3d6d8 }
.alert-secondary .alert-link{ color:#34383c }

.alert-success{ --bs-alert-color:#0f5132;--bs-alert-bg:#d1e7dd;--bs-alert-border-color:#badbcc }
.alert-success .alert-link{ color:#0c4128 }

.alert-info{ --bs-alert-color:#055160;--bs-alert-bg:#cff4fc;--bs-alert-border-color:#b6effb }
.alert-info .alert-link{ color:#04414d }

.alert-warning{ --bs-alert-color:#664d03;--bs-alert-bg:#fff3cd;--bs-alert-border-color:#ffecb5 }
.alert-warning .alert-link{ color:#523e02 }

.alert-danger{ --bs-alert-color:#842029;--bs-alert-bg:#f8d7da;--bs-alert-border-color:#f5c2c7 }
.alert-danger .alert-link{ color:#6a1a21 }

.alert-light{ --bs-alert-color:#636464;--bs-alert-bg:#fefefe;--bs-alert-border-color:#fdfdfe }
.alert-light .alert-link{ color:#4f5050 }

.alert-dark{ --bs-alert-color:#141619;--bs-alert-bg:#d3d3d4;--bs-alert-border-color:#bcbebf }
.alert-dark .alert-link{ color:#101214 }

template

<div style="--bs-alert-bg:transparent;--bs-alert-padding-x:1rem;--bs-alert-padding-y:1rem;--bs-alert-margin-bottom:1rem;--bs-alert-color:inherit;--bs-alert-border-color:transparent;--bs-alert-border:1px solid var(--bs-alert-border-color);--bs-alert-border-radius:0.375rem;position:relative;padding:var(--bs-alert-padding-y) var(--bs-alert-padding-x);margin-bottom:var(--bs-alert-margin-bottom);color:var(--bs-alert-color);background-color:var(--bs-alert-bg);border:var(--bs-alert-border);border-radius:var(--bs-alert-border-radius); padding-right:3rem;">
  <h4 style="color:inherit; margin-top:0;margin-bottom:.5rem;font-weight:500;line-height:1.2">X Alert !</h4>
  This is a x alert with <a href="#" style="font-weight:700">an example link</a>. Give it a click if you like.
</div>

bootstrap 5.x :root

:root{
  --bs-blue:#0d6efd;
  --bs-indigo:#6610f2;
  --bs-purple:#6f42c1;
  --bs-pink:#d63384;
  --bs-red:#dc3545;
  --bs-orange:#fd7e14;
  --bs-yellow:#ffc107;
  --bs-green:#198754;
  --bs-teal:#20c997;
  --bs-cyan:#0dcaf0;
  --bs-black:#000;
  --bs-white:#fff;
  --bs-gray:#6c757d;
  --bs-gray-dark:#343a40;
  --bs-gray-100:#f8f9fa;
  --bs-gray-200:#e9ecef;
  --bs-gray-300:#dee2e6;
  --bs-gray-400:#ced4da;
  --bs-gray-500:#adb5bd;
  --bs-gray-600:#6c757d;
  --bs-gray-700:#495057;
  --bs-gray-800:#343a40;
  --bs-gray-900:#212529;
  --bs-primary:#0d6efd;
  --bs-secondary:#6c757d;
  --bs-success:#198754;
  --bs-info:#0dcaf0;
  --bs-warning:#ffc107;
  --bs-danger:#dc3545;
  --bs-light:#f8f9fa;
  --bs-dark:#212529;
  --bs-primary-rgb:13,110,253;
  --bs-secondary-rgb:108,117,125;
  --bs-success-rgb:25,135,84;
  --bs-info-rgb:13,202,240;
  --bs-warning-rgb:255,193,7;
  --bs-danger-rgb:220,53,69;
  --bs-light-rgb:248,249,250;
  --bs-dark-rgb:33,37,41;
  --bs-white-rgb:255,255,255;
  --bs-black-rgb:0,0,0;
  --bs-body-color-rgb:33,37,41;
  --bs-body-bg-rgb:255,255,255;
  --bs-font-sans-serif:system-ui,-apple-system,"Segoe UI",Roboto,"Helvetica Neue","Noto Sans","Liberation Sans",Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol","Noto Color Emoji";
  --bs-font-monospace:SFMono-Regular,Menlo,Monaco,Consolas,"Liberation Mono","Courier New",monospace;
  --bs-gradient:linear-gradient(180deg, rgba(255, 255, 255, 0.15), rgba(255, 255, 255, 0));
  --bs-body-font-family:var(--bs-font-sans-serif);
  --bs-body-font-size:1rem;
  --bs-body-font-weight:400;
  --bs-body-line-height:1.5;
  --bs-body-color:#212529;
  --bs-body-bg:#fff;
  --bs-border-width:1px;
  --bs-border-style:solid;
  --bs-border-color:#dee2e6;
  --bs-border-color-translucent:rgba(0, 0, 0, 0.175);
  --bs-border-radius:0.375rem;
  --bs-border-radius-sm:0.25rem;
  --bs-border-radius-lg:0.5rem;
  --bs-border-radius-xl:1rem;
  --bs-border-radius-2xl:2rem;
  --bs-border-radius-pill:50rem;
  --bs-link-color:#0d6efd;
  --bs-link-hover-color:#0a58ca;
  --bs-code-color:#d63384;
  --bs-highlight-bg:#fff3cd
}

warning

5.x

<div style="--bs-alert-bg:transparent;--bs-alert-padding-x:1rem;--bs-alert-padding-y:1rem;--bs-alert-margin-bottom:1rem;--bs-alert-color:inherit;--bs-alert-border-color:transparent;--bs-alert-border:1px solid var(--bs-alert-border-color);--bs-alert-border-radius:0.375rem;position:relative;padding:var(--bs-alert-padding-y) var(--bs-alert-padding-x);margin-bottom:var(--bs-alert-margin-bottom);color:var(--bs-alert-color);background-color:var(--bs-alert-bg);border:var(--bs-alert-border);border-radius:var(--bs-alert-border-radius); padding-right:3rem; -bs-alert-color:#664d03;--bs-alert-bg:#fff3cd;--bs-alert-border-color:#ffecb5">
  <h4 style="color:inherit; margin-top:0;margin-bottom:.5rem;font-weight:500;line-height:1.2">Warning Alert !</h4>
  This is a warning alert with <a href="#" style="font-weight:700; color:#523e02">an example link</a>. Give it a click if you like.
</div>

original

<div class="alert alert-warning alert-dismissible fade show">
  <h4 class="alert-heading">Warning Alert !</h4>
  This is a warning alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>

4.x

<div style="position:relative; padding:.75rem 1.25rem; margin-bottom:1rem; border:1px solid transparent; border-radius:.25rem; color:#856404; background-color:#fff3cd; border-color:#ffeeba">
  <h4 style="font-size: 1.5rem; color:inherit">Warning Alert !</h4>
  This is a warning alert with <a href="#" style="color:#533f03">an example link</a>. Give it a click if you like.
</div>

<!-- 5.x colors -->
<div style="position:relative; padding:1rem 1rem; margin-bottom:1rem; border:1px solid transparent; border-radius:.375rem; padding-right:3rem; color:#664d03; background-color:#fff3cd; border-color:#ffecb5">
  <h4 style="font-size: 1.5rem; color: #523e02; margin-top:0;margin-bottom:.5rem;font-weight:500;line-height:1.2">Warning Alert !</h4>
  This is a warning alert with <a href="#" style="font-weight:700; color:#523e02">an example link</a>. Give it a click if you like.
</div>

original

<div class="alert alert-warning" role="alert">
  <h4 class="alert-heading">Warning Alert !</h4>
  This is a warning alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>

success

5.x

<div style="--bs-alert-bg:transparent;--bs-alert-padding-x:1rem;--bs-alert-padding-y:1rem;--bs-alert-margin-bottom:1rem;--bs-alert-color:inherit;--bs-alert-border-color:transparent;--bs-alert-border:1px solid var(--bs-alert-border-color);--bs-alert-border-radius:0.375rem;position:relative;padding:var(--bs-alert-padding-y) var(--bs-alert-padding-x);margin-bottom:var(--bs-alert-margin-bottom);color:var(--bs-alert-color);background-color:var(--bs-alert-bg);border:var(--bs-alert-border);border-radius:var(--bs-alert-border-radius); padding-right:3rem; --bs-alert-color:#0f5132;--bs-alert-bg:#d1e7dd;--bs-alert-border-color:#badbcc">
  <h4 style="color:inherit; margin-top:0;margin-bottom:.5rem;font-weight:500;line-height:1.2">Success Alert !</h4>
  This is a success alert with <a href="#" style="font-weight:700; color:#0c4128">an example link</a>. Give it a click if you like.
</div>

original

<div class="alert alert-success alert-dismissible fade show">
  <h4 class="alert-heading">Success Alert !</h4>
  This is a success alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>

4.x

<div style="position:relative; padding:.75rem 1.25rem; margin-bottom:1rem; border:1px solid transparent; border-radius:.25rem; color:#155724; background-color:#d4edda; border-color:#c3e6cb">
  <h4 style="color:inherit">Success Alert !</h4>
  This is a success alert with <a href="#" style="color:#0b2e13">an example link</a>. Give it a click if you like.
</div>

<!-- 5.x color -->
<div style="position:relative; padding:1rem 1rem; margin-bottom:1rem; border:1px solid transparent; border-radius:.375rem; padding-right:3rem; color:#0f5132; background-color:#d1e7dd; border-color:#badbcc">
  <h4 style="font-size: 1.5rem; color: inherit; margin-top:0;margin-bottom:.5rem;font-weight:500;line-height:1.2">Success Alert !</h4>
  This is a success alert with <a href="#" style="font-weight:700; color:#0c4128">an example link</a>. Give it a click if you like.
</div>

original

<div class="alert alert-success" role="alert">
  <h4 class="alert-heading">Success Alert !</h4>
  This is a success alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>

danger

5.x

<div style="--bs-alert-bg:transparent;--bs-alert-padding-x:1rem;--bs-alert-padding-y:1rem;--bs-alert-margin-bottom:1rem;--bs-alert-color:inherit;--bs-alert-border-color:transparent;--bs-alert-border:1px solid var(--bs-alert-border-color);--bs-alert-border-radius:0.375rem;position:relative;padding:var(--bs-alert-padding-y) var(--bs-alert-padding-x);margin-bottom:var(--bs-alert-margin-bottom);color:var(--bs-alert-color);background-color:var(--bs-alert-bg);border:var(--bs-alert-border);border-radius:var(--bs-alert-border-radius); padding-right:3rem; --bs-alert-color:#842029;--bs-alert-bg:#f8d7da;--bs-alert-border-color:#f5c2c7">
  <h4 style="color:inherit; margin-top:0;margin-bottom:.5rem;font-weight:500;line-height:1.2">Danger Alert !</h4>
  This is a danger alert with <a href="#" style="font-weight:700; color:#6a1a21">an example link</a>. Give it a click if you like.
</div>

original

<div class="alert alert-danger alert-dismissible fade show">
  <h4 class="alert-heading">Danger Alert !</h4>
  This is a danger alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>

4.x

<div style="position:relative; padding:.75rem 1.25rem; margin-bottom:1rem; border:1px solid transparent; border-radius:.25rem; color:#721c24; background-color:#f8d7da; border-color:#f5c6cb">
  <h4 style="font-size: 1.5rem; color:inherit">Danger Alter !</h4>
  This is a danger alert with <a href="#" style="color:#491217">an example link</a>. Give it a click if you like.
</div>

<!-- 5.x color -->
<div style="position:relative; padding:1rem 1rem; margin-bottom:1rem; border:1px solid transparent; border-radius:.375rem; color:#842029; background-color:#f8d7da; border-color:#f5c2c7">
  <h4 style="font-size: 1.5rem; color:inherit; margin-top:0;margin-bottom:.5rem;font-weight:500;line-height:1.2">Danger Alert !</h4>
  This is a danger alert with <a href="#" style="font-weight:700; color:#6a1a21;">an example link</a>. Give it a click if you like.
</div>

original

<div class="alert alert-danger" role="alert">
  <h4 class="alert-heading">Danger Alert !</h4>
  This is a danger alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>

primary

5.x

<div style="--bs-alert-bg:transparent;--bs-alert-padding-x:1rem;--bs-alert-padding-y:1rem;--bs-alert-margin-bottom:1rem;--bs-alert-color:inherit;--bs-alert-border-color:transparent;--bs-alert-border:1px solid var(--bs-alert-border-color);--bs-alert-border-radius:0.375rem;position:relative;padding:var(--bs-alert-padding-y) var(--bs-alert-padding-x);margin-bottom:var(--bs-alert-margin-bottom);color:var(--bs-alert-color);background-color:var(--bs-alert-bg);border:var(--bs-alert-border);border-radius:var(--bs-alert-border-radius); padding-right:3rem; --bs-alert-color:#084298;--bs-alert-bg:#cfe2ff;--bs-alert-border-color:#b6d4fe">
  <h4 style="color:inherit; margin-top:0;margin-bottom:.5rem;font-weight:500;line-height:1.2">Primary Alert !</h4>
  This is a primary alert with <a href="#" style="font-weight:700; color:#06357a">an example link</a>. Give it a click if you like.
</div>

original

<div class="alert alert-primary alert-dismissible fade show">
  <h4 class="alert-heading">Primary Alert !</h4>
  This is a primary alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>

4.x

<div style="position:relative; padding:.75rem 1.25rem; margin-bottom:1rem; border:1px solid transparent; border-radius:.25rem; color:#004085; background-color:#cce5ff; border-color:#b8daff">
  <h4 style="font-size: 1.5rem; color:inherit">Primary Alert !</h4>
  This is a primary alert with <a href="#" style="color:#002752">an example link</a>. Give it a click if you like.
</div>

<!-- 5.x color -->
<div style="position:relative; padding:1rem 1rem; margin-bottom:1rem; border:1px solid transparent; border-radius:.375rem; padding-right:3rem; color:#084298; background-color:#cfe2ff; border-color:#b6d4fe">
  <h4 style="font-size: 1.5rem; color:inherit; margin-top:0;margin-bottom:.5rem;font-weight:500;line-height:1.2">Primary Alert !</h4>
  This is a primary alert with <a href="#" style="font-weight:700; color:#06357a">an example link</a>. Give it a click if you like.
</div>

original

<div class="alert alert-primary" role="alert">
  <h4 class="alert-heading">Primary Alert !</h4>
  This is a primary alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>

info

5.x

<div style="--bs-alert-bg:transparent;--bs-alert-padding-x:1rem;--bs-alert-padding-y:1rem;--bs-alert-margin-bottom:1rem;--bs-alert-color:inherit;--bs-alert-border-color:transparent;--bs-alert-border:1px solid var(--bs-alert-border-color);--bs-alert-border-radius:0.375rem;position:relative;padding:var(--bs-alert-padding-y) var(--bs-alert-padding-x);margin-bottom:var(--bs-alert-margin-bottom);color:var(--bs-alert-color);background-color:var(--bs-alert-bg);border:var(--bs-alert-border);border-radius:var(--bs-alert-border-radius); padding-right:3rem; --bs-alert-color:#055160;--bs-alert-bg:#cff4fc;--bs-alert-border-color:#b6effb">
  <h4 style="color:inherit; margin-top:0;margin-bottom:.5rem;font-weight:500;line-height:1.2">Info Alert !</h4>
  This is a info alert with <a href="#" style="font-weight:700; color:#04414d">an example link</a>. Give it a click if you like.
</div>

original

<div class="alert alert-info alert-dismissible fade show">
  <h4 class="alert-heading">Info Alert !</h4>
  This is a info alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>

4.x

<div style="position:relative; padding:.75rem 1.25rem; margin-bottom:1rem; border:1px solid transparent; border-radius:.25rem; color:#0c5460;background-color:#d1ecf1;border-color:#bee5eb">
  <h4 style="font-size: 1.5rem; color:inherit">Info Alter !</h4>
  This is a info alert with <a href="#" style="color:#062c33">an example link</a>. Give it a click if you like.
</div>

<!-- 4.x style 5.x color -->
<div style="position:relative; padding:1rem 1rem; margin-bottom:1rem; border:1px solid transparent; border-radius:.375rem; padding-right:3rem; color:#055160; background-color:#cff4fc; border-color:#b6effb">
  <h4 style="font-size: 1.5rem; color:inherit; margin-top:0;margin-bottom:.5rem;font-weight:500;line-height:1.2">Info Alert !</h4>
  This is a info alert with <a href="#" style="font-weight:700; color:#04414d">an example link</a>. Give it a click if you like.
</div>

original

<div class="alert alert-info" role="alert">
  <h4 class="alert-heading">Info Alert !</h4>
  This is a info alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>

callout

references:

callout

.bd-callout { padding: 1.25rem; margin-top: 1.25rem; margin-bottom: 1.25rem; border: 1px solid #eee; border-left-width: .25rem; border-radius: .25rem }
.bd-callout h4 { margin-top: 0; margin-bottom: .25rem }

.bd-callout p:last-child { margin-bottom: 0 }
.bd-callout code { border-radius: .25rem }
.bd-callout+.bd-callout { margin-top: -.25rem }

.bd-callout-info { border-left-color: #5bc0de }
.bd-callout-info h4 { color: #5bc0de }

.bd-callout-warning { border-left-color: #f0ad4e }
.bd-callout-warning h4 { color: #f0ad4e }

.bd-callout-danger { border-left-color: #d9534f }
.bd-callout-danger h4 { color: #d9534f }

.bd-callout-primary{ border-left-color: #007bff }
.bd-callout-primary h4 { color: #007bff }

.bd-callout-success{ border-left-color: #28a745 }
.bd-callout-success h4 { color: #28a745 }

.bd-callout-default{ border-left-color: #6c757d }
.bd-callout-default h4 { color: #6c757d }

default

<div style="display: block; padding: 1.25rem; margin-top: 1.25rem; margin-bottom: 1.25rem; border: 1px solid #eee; border-left-width: 0.25rem; border-radius: 0.25rem; border-left-color: #6c757d;">
  <h4 style="margin-top: 0; margin-bottom: 0.25rem; color: #6c757d;">Default Callout</h4>
  This is a default callout.
</div>

original

<div class="bd-callout bd-callout-default">
  <h4>Default Callout</h4>
  This is a default callout.
</div>

primary

<div style="display: block; padding: 1.25rem; margin-top: 1.25rem; margin-bottom: 1.25rem; border: 1px solid #eee; border-left-width: .25rem; border-radius: .25rem; border-left-color: #007bff">
  <h4 style="margin-top: 0; margin-bottom: 0.25rem; color: #007bff">Primary Callout</h4>
  This is a primary callout.
</div>

original

<div class="bd-callout bd-callout-primary">
  <h4>Primary Callout</h4>
  This is a primary callout.
</div>

warning

<div style="display: block; padding: 1.25rem; margin-top: 1.25rem; margin-bottom: 1.25rem; border: 1px solid #eee; border-left-width: .25rem; border-radius: .25rem; border-left-color: #f0ad4e">
  <h4 style="margin-top: 0; margin-bottom: 0.25rem; color: #f0ad4e">Warning Callout</h4>
  This is a warning callout.
</div>

original

<div class="bd-callout bd-callout-warning">
  <h4>Warning Callout</h4>
  This is a warning callout.
</div>

danger

<div style="display: block; padding: 1.25rem; margin-top: 1.25rem; margin-bottom: 1.25rem; border: 1px solid #eee; border-left-width: .25rem; border-radius: .25rem; border-left-color: #d9534f">
  <h4 style="margin-top: 0; margin-bottom: 0.25rem; color: #d9534f">Danger Callout</h4>
  This is a danger callout.
</div>

original

<div class="bd-callout bd-callout-danger">
  <h4>Danger Callout</h4>
  This is a danger callout.
</div>

succeed

<div style="display: block; padding: 1.25rem; margin-top: 1.25rem; margin-bottom: 1.25rem; border: 1px solid #eee; border-left-width: .25rem; border-radius: .25rem; border-left-color: #28a745">
  <h4 style="margin-top: 0; margin-bottom: 0.25rem; color: #28a745">Succeed Callout</h4>
  This is a succeed callout.
</div>

original

<div class="bd-callout bd-callout-success">
  <h4>Success Callout</h4>
  This is a success callout.
</div>

info

<div style="display: block; padding: 1.25rem; margin-top: 1.25rem; margin-bottom: 1.25rem; border: 1px solid #eee; border-left-width: .25rem; border-radius: .25rem; border-left-color: #5bc0de">
  <h4 style="margin-top: 0; margin-bottom: 0.25rem; color: #5bc0de">Info Callout</h4>
  This is info callout.
</div>

original

<div class="bd-callout bd-callout-info">
  <h4>Info Callout</h4>
  This is an info callout.
</div>

kbd

references:

kbd {
  margin: 0px 0.1em;
  padding: 0.1em 0.6em;
  border-radius: 3px;
  border: 1px solid #ccc;
  color: #333;
  line-height: 1.4;
  font-size: 12px;
  display: inline-block;
  box-shadow: 0px 1px 0px rgba(0,0,0,0.2), inset 0px 0px 0px 2px #ffffff;
  background-color: rgb(247, 247, 247);
  text-shadow: 0 1px 0 #fff;
  -moz-box-shadow: 0 1px 0px rgba(0, 0, 0, 0.2), 0 0 0 2px #ffffff inset;
  -webkit-box-shadow: 0 1px 0px rgba(0, 0, 0, 0.2), 0 0 0 2px #ffffff inset;
  -moz-border-radius: 3px;
  -webkit-border-radius: 3px;
}

kbd {
  font-family:SFMono-Regular,Menlo,Monaco,Consolas,"Liberation Mono","Courier New",monospace;
  font-size:1em;
  padding:.2rem .4rem;
  font-size:87.5%;
  color:#fff;
  background-color:#212529;
  border-radius:.2rem
}
kbd kbd {
  padding:0;
  font-size:100%;
  font-weight:700;
}

kbd {
  padding: 2px 4px;
  font-size: 90%;
  color: #fff;
  background-color: #333;
  border-radius: 3px;
  -webkit-box-shadow: inset 0 -1px 0 rgb(0 0 0 / 25%);
  box-shadow: inset 0 -1px 0 rgb(0 0 0 / 25%);
}

or for html

<a href="#" style="margin:0px 0.1em;padding:0.1em 0.6em;border-radius:3px;border:1px solid #ccc;color:#333;line-height:1.4;font-size:12px;display:inline-block;box-shadow:0px 1px 0px rgba(0,0,0,0.2), inset 0px 0px 0px 2px #ffffff;background-color:rgb(247, 247, 247);text-shadow:0 1px 0 #fff;">button</a>

<!-- or -->
<a href="#" style="margin:0px 0.1em;padding:0.1em 0.6em;border-radius:3px;border:1px solid #ccc;box-shadow:0px 1px 0px rgba(0,0,0,0.2), inset 0px 0px 0px 2px #ffffff;text-shadow:0 1px 0 #fff;line-height:1.4;font-size:12.0px;background-color:rgb(247,247,247);display:inline-block;color:rgb(51,51,51);">click me</a>

border

[!NOTE|label:references:]

padding

/*       + top, right, bottom, left   */
padding: 1px;

/*        + top, bottom               */
/*        |  + right, left            */
padding: 1px 2px;

/*        + top                       */
/*        |  + right, left            */
/*        |  |   + bottom             */
padding: 1px 2px 3px;

/*        + top                       */
/*        |  + right                  */
/*        |  |   + bottom             */
/*        |  |   |   + left           */
padding: 1px 2px 3px 4px;

border-style

/*            + top, right, bottom, left */
border-style: dotted;

/*            + top, bottom              */
/*            |      + right, left       */
border-style: dotted dashed;

/*            + top                      */
/*            |      + right, left       */
/*            |      |      + bottom     */
border-style: dotted dashed solid;

/*            + top                      */
/*            |      + right             */
/*            |      |      + bottom     */
/*            |      |      |     + left */
border-style: dotted dashed solid groove;

font

references:
best fonts for developer
find fonts via image

selector

[!TIP|references:references] references:
: .name
: #name
: [attr], [attr=value], [attr~=value], [attr|=value], [attr^=value], [attr$=value], [attr*=value]
: ul > li
: p ~ span
: h2 + p
: col || td
: p::first-line
[Pseudo-classes] (https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-classes)
:root
:first-child
:last-child
:hover
:active

attribute selectors

[attr] : Represents elements with an attribute name of attr.
[attr=value] : Represents elements with an attribute name of attr whose value is exactly value.
[attr~=value] : Represents elements with an attribute name of attr whose value is a whitespace-separated list of words, one of which is exactly value.
[attr|=value] : Represents elements with an attribute name of attr whose value can be exactly value or can begin with value immediately followed by a hyphen, - (U+002D). It is often used for language subcode matches.
[attr^=value] : Represents elements with an attribute name of attr whose value is prefixed (preceded) by value.
[attr$=value] : Represents elements with an attribute name of attr whose value is suffixed (followed) by value.
[attr*=value] : Represents elements with an attribute name of attr whose value contains at least one occurrence of value within the string.

example

<ul>
  <li><a href="#internal">Internal link</a></li>
  <li><a href="http://example.com">Example link</a></li>
  <li><a href="#InSensitive">Insensitive internal link</a></li>
  <li><a href="http://example.org">Example org link</a></li>
  <li><a href="https://example.org">Example https org link</a></li>
</ul>

a { color: blue; }

/* Internal links, beginning with "#" */
a[href^="#"] { background-color: gold; }

/* Links with "example" anywhere in the URL */
a[href*="example"] { background-color: silver; }

/* Links with "insensitive" anywhere in the URL, regardless of capitalization */
a[href*="insensitive" i] { color: cyan; }

/* Links with "cAsE" anywhere in the URL, with matching capitalization */
a[href*="cAsE" s] { color: pink; }

/* Links that end in ".org" */
a[href$=".org"] { color: red; }

/* Links that start with "https" and end in ".org" */
a[href^="https"][href$=".org"] { color: green; }

another example

sample code

.dh-tl-tl {}
.dh-tl-tr {}
.dh-tl-br {}
.dh-tl-bl {}
.dh-tr-tl {}
.dh-br-tl {}
.dh-bl-tl {}

elegant way

[class^="dh-"][class$="-tl"] { color: red;    }
[class^="dh-"][class$="-tr"] { color: orange; }
[class^="dh-"][class$="-bl"] { color: green;  }
[class^="dh-"][class$="-br"] { color: blue;   }

debug tooltips in inspect

[!NOTE|label:references:]

select the element in the inspector
select Event-Liseners tab in right panel
delete the mouseover event
re-hover the element in web page

[!NOTE|label:references:]

sample html:

<div class="Popover js-hovercard-content position-absolute" style="display: none; outline: none; left: 956.862px; z-index: 100; top: 426.119px !important; bottom: auto !important; opacity: 1;" aria-label="Hovercard" role="region">
  <div class="Popover-message Popover-message--large Box color-shadow-large Popover-message--top-right" style="width: 360px; display: block; opacity: 1; visibility: visible;"></div>
</div>

⌥ + ⌘ + i to open the DevTools
go to Console tab
paste and execute following javascript code

// 1. 监听并阻止 `mouseleave` 事件，防止 Popover 被 GitHub 隐藏
document.querySelectorAll('.Popover').forEach(el => {
    el.addEventListener('mouseleave', e => e.stopPropagation(), true);
});

// 2. 监听 DOM 变化，防止 Popover 被 GitHub 代码修改 `display`
const observer = new MutationObserver(mutations => {
    mutations.forEach(mutation => {
        mutation.target.querySelectorAll('.Popover').forEach(popover => {
            if (popover.style.display === 'none') {
                console.log('Detected hidden Popover, restoring...');
                popover.style.display = 'block';
                popover.style.opacity = '1';
                popover.style.visibility = 'visible';
            }
        });
    });
});

// 开始监听整个页面的 DOM 变化
observer.observe(document.body, { childList: true, subtree: true });

// 3. 持续检查 `Popover` 是否被 GitHub 代码隐藏
setInterval(() => {
    document.querySelectorAll('.Popover').forEach(popover => {
        if (popover.style.display === 'none') {
            console.log('Forcing Popover to be visible...');
            popover.style.display = 'block';
            popover.style.opacity = '1';
            popover.style.visibility = 'visible';
        }
    });
}, 100);

references

SELECTOR

EXAMPLE

EXAMPLE DESCRIPTION

.class

.intro

Selects all elements with class="intro"

.class1.class2

.name1.name2

Selects all elements with both name1 and name2 set within its class attribute

.class1 .class2

.name1 .name2

Selects all elements with name2 that is a descendant of an element with name1

#id

#firstname

Selects the element with id="firstname"

*

Selects all elements

element

p

Selects all <p> elements

element.class

p.intro

Selects all <p> elements with class="intro"

element,element

div, p

Selects all <div> elements and all <p> elements

element element

div p

Selects all <p> elements inside <div> elements

element>element

div > p

Selects all <p> elements where the parent is a <div> element

element+element

div + p

Selects the first <p> element that is placed immediately after <div> elements

element1~element2

p ~ ul

Selects every <ul> element that is preceded by a <p> element

[attribute]

[target]

Selects all elements with a target attribute

[attribute=value]

[target=_blank]

Selects all elements with target="_blank"

[attribute~=value]

[title~=flower]

Selects all elements with a title attribute containing the word "flower"

[attribute∣=value]

[lang∣=en]

Selects all elements with a lang attribute value equal to "en" or starting with "en-"

[attribute^=value]

a[href^="https"]

Selects every <a> element whose href attribute value begins with "https"

[attribute$=value]

a[href$=".pdf"]

Selects every <a> element whose href attribute value ends with ".pdf"

[attribute*=value]

a[href*="w3schools"]

Selects every <a> element whose href attribute value contains the substring "w3schools"

:active

a:active

Selects the active link

::after

p::after

Insert something after the content of each <p> element

::before

p::before

Insert something before the content of each <p> element

:checked

input:checked

Selects every checked <input> element

:default

input:default

Selects the default <input> element

:disabled

input:disabled

Selects every disabled <input> element

:empty

p:empty

Selects every <p> element that has no children (including text nodes)

:enabled

input:enabled

Selects every enabled <input> element

:first-child

p:first-child

Selects every <p> element that is the first child of its parent

::first-letter

p::first-letter

Selects the first letter of every <p> element

::first-line

p::first-line

Selects the first line of every <p> element

:first-of-type

p:first-of-type

Selects every <p> element that is the first <p> element of its parent

:focus

input:focus

Selects the input element which has focus

:fullscreen

Selects the element that is in full-screen mode

:hover

a:hover

Selects links on mouse over

:in-range

input:in-range

Selects input elements with a value within a specified range

:indeterminate

input:indeterminate

Selects input elements that are in an indeterminate state

:invalid

input:invalid

Selects all input elements with an invalid value

:lang(language)

p:lang(it)

Selects every <p> element with a lang attribute equal to "it" (Italian)

:last-child

p:last-child

Selects every <p> element that is the last child of its parent

:last-of-type

p:last-of-type

Selects every <p> element that is the last <p> element of its parent

:link

a:link

Selects all unvisited links

::marker

Selects the markers of list items

:not(selector)

:not(p)

Selects every element that is not a <p> element

:nth-child(n)

p:nth-child(2)

Selects every <p> element that is the second child of its parent

:nth-last-child(n)

p:nth-last-child(2)

Selects every <p> element that is the second child of its parent, counting from the last child

:nth-last-of-type(n)

p:nth-last-of-type(2)

Selects every <p> element that is the second <p> element of its parent, counting from the last child

:nth-of-type(n)

p:nth-of-type(2)

Selects every <p> element that is the second <p> element of its parent

:only-of-type

p:only-of-type

Selects every <p> element that is the only <p> element of its parent

:only-child

p:only-child

Selects every <p> element that is the only child of its parent

:optional

input:optional

Selects input elements with no "required" attribute

:out-of-range

input:out-of-range

Selects input elements with a value outside a specified range

::placeholder

input::placeholder

Selects input elements with the "placeholder" attribute specified

:read-only

input:read-only

Selects input elements with the "readonly" attribute specified

:read-write

input:read-write

Selects input elements with the "readonly" attribute NOT specified

:required

input:required

Selects input elements with the "required" attribute specified

:root

Selects the document's root element

::selection

Selects the portion of an element that is selected by a user

:target

#news:target

Selects the current active #news element (clicked on a URL containing that anchor name)

stylus

tricky

using bas64 string for svg

system font stack

bootstrap

using boostrap sytles

alert

bootstrap alert

callout

kbd

border

padding

border-style

font

selector

attribute selectors

debug tooltips in inspect

debug Popover in github

references

debug `Popover` in github