feat(html): add class helper

Co-Authored-By: Virgil <virgil@lethean.io>

docs/architecture.md

@@ -42,6 +42,7 @@ Accessibility-oriented helpers are also provided for common attribute patterns:
 - `Lang(node, locale)`
 - `Dir(node, direction)`
 - `Alt(node, text)`
+- `Class(node, classes...)`
 - `AriaHidden(node, hidden)`
 - `AriaExpanded(node, expanded)`
 - `TabIndex(node, index)`

node.go

@@ -228,6 +228,13 @@ func Alt(n Node, text string) Node {
 	return Attr(n, "alt", text)
 }
 
+// node.go: Class sets the class attribute on an element node.
+// Example: Class(El("div"), "card", "card--primary").
+// Multiple class tokens are joined with spaces.
+func Class(n Node, classes ...string) Node {
+	return Attr(n, "class", strings.Join(classes, " "))
+}
+
 // node.go: AriaHidden sets the aria-hidden attribute on an element node.
 // Example: AriaHidden(El("svg"), true).
 func AriaHidden(n Node, hidden bool) Node {

node_test.go

@@ -324,6 +324,16 @@ func TestAltHelper(t *testing.T) {
 	}
 }
 
+func TestClassHelper(t *testing.T) {
+	ctx := NewContext()
+	node := Class(El("div", Raw("content")), "card", "card--primary")
+	got := node.Render(ctx)
+	want := `<div class="card card--primary">content</div>`
+	if got != want {
+		t.Errorf("Class() = %q, want %q", got, want)
+	}
+}
+
 func TestAriaHiddenHelper(t *testing.T) {
 	ctx := NewContext()