Refactor backend SVG package and add tests (#26335)

Introduce a well-tested `svg.Normalize` function.
Make `RenderHTML` faster and more stable.

modules/html/html.go

@@ -6,28 +6,20 @@ package html
 // ParseSizeAndClass get size and class from string with default values
 // If present, "others" expects the new size first and then the classes to use
 func ParseSizeAndClass(defaultSize int, defaultClass string, others ...any) (int, string) {
-	if len(others) == 0 {
-		return defaultSize, defaultClass
-	}
-
 	size := defaultSize
-	_size, ok := others[0].(int)
-	if ok && _size != 0 {
-		size = _size
-	}
-
-	if len(others) == 1 {
-		return size, defaultClass
-	}
-
-	class := defaultClass
-	if _class, ok := others[1].(string); ok && _class != "" {
-		if defaultClass == "" {
-			class = _class
-		} else {
-			class = defaultClass + " " + _class
+	if len(others) >= 1 {
+		if v, ok := others[0].(int); ok && v != 0 {
+			size = v
+		}
+	}
+	class := defaultClass
+	if len(others) >= 2 {
+		if v, ok := others[1].(string); ok && v != "" {
+			if class != "" {
+				class += " "
+			}
+			class += v
 		}
 	}
-
 	return size, class
 }