Fix markdown color code detection (#30208)

When reviewing PRs, some color names might be mentioned, the
`transformCodeSpan` (which calls `css.ColorHandler`) considered it as a
valid color, but actually it shouldn't be rendered as a color codespan.

modules/markup/markdown/markdown_test.go

@@ -436,6 +436,10 @@ func TestColorPreview(t *testing.T) {
 		testcase string
 		expected string
 	}{
+		{ // do not render color names
+			"The CSS class `red` is there",
+			"<p>The CSS class <code>red</code> is there</p>\n",
+		},
 		{ // hex
 			"`#FF0000`",
 			`<p><code>#FF0000<span class="color-preview" style="background-color: #FF0000"></span></code></p>` + nl,
@@ -445,8 +449,8 @@ func TestColorPreview(t *testing.T) {
 			`<p><code>rgb(16, 32, 64)<span class="color-preview" style="background-color: rgb(16, 32, 64)"></span></code></p>` + nl,
 		},
 		{ // short hex
-			"This is the color white `#000`",
-			`<p>This is the color white <code>#000<span class="color-preview" style="background-color: #000"></span></code></p>` + nl,
+			"This is the color white `#0a0`",
+			`<p>This is the color white <code>#0a0<span class="color-preview" style="background-color: #0a0"></span></code></p>` + nl,
 		},
 		{ // hsl
 			"HSL stands for hue, saturation, and lightness. An example: `hsl(0, 100%, 50%)`.",

modules/markup/markdown/transform_codespan.go

@@ -49,9 +49,28 @@ func (r *HTMLRenderer) renderCodeSpan(w util.BufWriter, source []byte, n ast.Nod
 	return ast.WalkContinue, nil
 }
 
+// cssColorHandler checks if a string is a render-able CSS color value.
+// The code is from "github.com/microcosm-cc/bluemonday/css.ColorHandler", except that it doesn't handle color words like "red".
+func cssColorHandler(value string) bool {
+	value = strings.ToLower(value)
+	if css.HexRGB.MatchString(value) {
+		return true
+	}
+	if css.RGB.MatchString(value) {
+		return true
+	}
+	if css.RGBA.MatchString(value) {
+		return true
+	}
+	if css.HSL.MatchString(value) {
+		return true
+	}
+	return css.HSLA.MatchString(value)
+}
+
 func (g *ASTTransformer) transformCodeSpan(ctx *markup.RenderContext, v *ast.CodeSpan, reader text.Reader) {
 	colorContent := v.Text(reader.Source())
-	if css.ColorHandler(strings.ToLower(string(colorContent))) {
+	if cssColorHandler(string(colorContent)) {
 		v.AppendChild(v, NewColorPreview(colorContent))
 	}
 }