html: add "in template" insertion mode support

See:
https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-intemplate

Updates golang/go#23071

Change-Id: I36529b7cf5d2adf159ed5c471fba9f67890b7eb9
Reviewed-on: https://go-review.googlesource.com/94838
Run-TryBot: Kunpei Sakai <namusyaka@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>

html/node.go

@@ -174,6 +174,16 @@ func (s *nodeStack) index(n *Node) int {
 	return -1
 }
 
+// contains returns whether a is within s.
+func (s *nodeStack) contains(a atom.Atom) bool {
+	for _, n := range *s {
+		if n.DataAtom == a {
+			return true
+		}
+	}
+	return false
+}
+
 // insert inserts a node at the given index.
 func (s *nodeStack) insert(i int, n *Node) {
 	(*s) = append(*s, nil)
@@ -192,3 +202,19 @@ func (s *nodeStack) remove(n *Node) {
 	(*s)[j] = nil
 	*s = (*s)[:j]
 }
+
+type insertionModeStack []insertionMode
+
+func (s *insertionModeStack) pop() (im insertionMode) {
+	i := len(*s)
+	im = (*s)[i-1]
+	*s = (*s)[:i-1]
+	return im
+}
+
+func (s *insertionModeStack) top() insertionMode {
+	if i := len(*s); i > 0 {
+		return (*s)[i-1]
+	}
+	return nil
+}

html/parse_test.go

@@ -125,6 +125,7 @@ func (a sortedAttributes) Swap(i, j int) {
 
 func dumpLevel(w io.Writer, n *Node, level int) error {
 	dumpIndent(w, level)
+	level++
 	switch n.Type {
 	case ErrorNode:
 		return errors.New("unexpected ErrorNode")
@@ -140,13 +141,19 @@ func dumpLevel(w io.Writer, n *Node, level int) error {
 		sort.Sort(attr)
 		for _, a := range attr {
 			io.WriteString(w, "\n")
-			dumpIndent(w, level+1)
+			dumpIndent(w, level)
 			if a.Namespace != "" {
 				fmt.Fprintf(w, `%s %s="%s"`, a.Namespace, a.Key, a.Val)
 			} else {
 				fmt.Fprintf(w, `%s="%s"`, a.Key, a.Val)
 			}
 		}
+		if n.Namespace == "" && n.DataAtom == atom.Template {
+			io.WriteString(w, "\n")
+			dumpIndent(w, level)
+			level++
+			io.WriteString(w, "content")
+		}
 	case TextNode:
 		fmt.Fprintf(w, `"%s"`, n.Data)
 	case CommentNode:
@@ -176,7 +183,7 @@ func dumpLevel(w io.Writer, n *Node, level int) error {
 	}
 	io.WriteString(w, "\n")
 	for c := n.FirstChild; c != nil; c = c.NextSibling {
-		if err := dumpLevel(w, c, level+1); err != nil {
+		if err := dumpLevel(w, c, level); err != nil {
 			return err
 		}
 	}