html: add PrependChild and InsertAfter

These methods complement the already existing AppendChild and
InsertBefore methods in allowing for easy and safe manipulation of
Node trees.

For example code like

oldChild.Parent.InsertBefore(newChild, oldChild.NextSibling)

can now be written like

oldChild.Parent.InsertAfter(newChild, oldChild)

and code like

node.InsertBefore(newNode, node.FirstChild)

can now be written like

node.PrependChild(newNode)

Fixes #46960

html/node.go

@@ -81,7 +81,37 @@ func (n *Node) InsertBefore(newChild, oldChild *Node) {
 	newChild.NextSibling = next
 }
 
-// AppendChild adds a node c as a child of n.
+// InsertAfter inserts newChild as a child of n, immediately after oldChild
+// in the sequence of n's children. oldChild may be nil, in which case newChild
+// is prepended to the beginning of n's children.
+//
+// It will panic if newChild already has a parent or siblings.
+func (n *Node) InsertAfter(newChild, oldChild *Node) {
+	if newChild.Parent != nil || newChild.PrevSibling != nil || newChild.NextSibling != nil {
+		panic("html: InsertAfter called for an attached child Node")
+	}
+	var prev, next *Node
+	if oldChild != nil {
+		prev, next = oldChild, oldChild.NextSibling
+	} else {
+		next = n.FirstChild
+	}
+	if next != nil {
+		next.PrevSibling = newChild
+	} else {
+		n.LastChild = newChild
+	}
+	if prev != nil {
+		prev.NextSibling = newChild
+	} else {
+		n.FirstChild = newChild
+	}
+	newChild.Parent = n
+	newChild.PrevSibling = prev
+	newChild.NextSibling = next
+}
+
+// AppendChild adds a node c as the last child of n.
 //
 // It will panic if c already has a parent or siblings.
 func (n *Node) AppendChild(c *Node) {
@@ -99,6 +129,24 @@ func (n *Node) AppendChild(c *Node) {
 	c.PrevSibling = last
 }
 
+// PrependChild adds a node c as the first child of n.
+//
+// It will panic if c already has a parent or siblings.
+func (n *Node) PrependChild(c *Node) {
+	if c.Parent != nil || c.PrevSibling != nil || c.NextSibling != nil {
+		panic("html: PrependChild called for an attached child Node")
+	}
+	first := n.FirstChild
+	if first != nil {
+		first.PrevSibling = c
+	} else {
+		n.LastChild = c
+	}
+	n.FirstChild = c
+	c.Parent = n
+	c.NextSibling = first
+}
+
 // RemoveChild removes a node c that is a child of n. Afterwards, c will have
 // no parent and no siblings.
 //