Add `.warning` class for doc author use

This is designed to be compatible with rustdoc's version, in
https://github.com/rust-lang/rust/pull/106561

guide/src/format/mdbook.md

@@ -314,3 +314,29 @@ contents (sidebar) by including a `\{{#title ...}}` near the top of the page.
 ```hbs
 \{{#title My Title}}
 ```
+
+## HTML classes provided by mdBook
+
+The `class="warning"` can be applied to make a warning, or similar note, stand out.
+
+```html
+<div class="warning">
+
+This is a bad thing that you should pay attention to.
+
+Warning blocks should be used sparingly in documentation, to avoid "warning
+fatique," where people are trained to ignore them because they usually don't
+matter for what they're doing.
+
+</div>
+```
+
+<div class="warning">
+
+This is a bad thing that you should pay attention to.
+
+Warning blocks should be used sparingly in documentation, to avoid "warning
+fatique," where people are trained to ignore them because they usually don't
+matter for what they're doing.
+
+</div>

src/theme/css/general.css

@@ -156,6 +156,29 @@ blockquote {
     border-block-end: .1em solid var(--quote-border);
 }
 
+.warning {
+    margin: 20px;
+    padding: 0 20px;
+    border-inline-start: 2px solid var(--warning-border);
+}
+
+.warning:before {
+    position: absolute;
+    width: 3rem;
+    height: 3rem;
+    margin-inline-start: calc(-1.5rem - 21px);
+    content: "ⓘ";
+    text-align: center;
+    background-color: var(--bg);
+    color: var(--warning-border);
+    font-weight: bold;
+    font-size: 2rem;
+}
+
+blockquote .warning:before {
+    background-color: var(--quote-bg);
+}
+
 kbd {
     background-color: var(--table-border-color);
     border-radius: 4px;

src/theme/css/variables.css

@@ -38,6 +38,8 @@
     --quote-bg: hsl(226, 15%, 17%);
     --quote-border: hsl(226, 15%, 22%);
 
+    --warning-border: #ff8e00;
+
     --table-border-color: hsl(210, 25%, 13%);
     --table-header-bg: hsl(210, 25%, 28%);
     --table-alternate-bg: hsl(210, 25%, 11%);
@@ -80,6 +82,8 @@
     --quote-bg: hsl(234, 21%, 18%);
     --quote-border: hsl(234, 21%, 23%);
 
+    --warning-border: #ff8e00;
+
     --table-border-color: hsl(200, 7%, 13%);
     --table-header-bg: hsl(200, 7%, 28%);
     --table-alternate-bg: hsl(200, 7%, 11%);
@@ -122,6 +126,8 @@
     --quote-bg: hsl(197, 37%, 96%);
     --quote-border: hsl(197, 37%, 91%);
 
+    --warning-border: #ff8e00;
+
     --table-border-color: hsl(0, 0%, 95%);
     --table-header-bg: hsl(0, 0%, 80%);
     --table-alternate-bg: hsl(0, 0%, 97%);
@@ -164,6 +170,8 @@
     --quote-bg: hsl(226, 15%, 17%);
     --quote-border: hsl(226, 15%, 22%);
 
+    --warning-border: #ff8e00;
+
     --table-border-color: hsl(226, 23%, 16%);
     --table-header-bg: hsl(226, 23%, 31%);
     --table-alternate-bg: hsl(226, 23%, 14%);
@@ -206,6 +214,8 @@
     --quote-bg: hsl(60, 5%, 75%);
     --quote-border: hsl(60, 5%, 70%);
 
+    --warning-border: #ff8e00;
+
     --table-border-color: hsl(60, 9%, 82%);
     --table-header-bg: #b3a497;
     --table-alternate-bg: hsl(60, 9%, 84%);
@@ -249,6 +259,8 @@
         --quote-bg: hsl(234, 21%, 18%);
         --quote-border: hsl(234, 21%, 23%);
 
+        --warning-border: #ff8e00;
+
         --table-border-color: hsl(200, 7%, 13%);
         --table-header-bg: hsl(200, 7%, 28%);
         --table-alternate-bg: hsl(200, 7%, 11%);