FindNodeByID searches the tree rooted at "node" for a node with the "id" attribute with the specified value.

GetAttr returns the value for an attribute, or "" if no such attribute is present.

GetAttrMap returns a map of attributes by name.

GetNodeTextContent returns the text content of a node.

GetNumericAttrFromMapWithDefault parses an attribute to a number, and returns the value, or returns `def` if the value is not present or cannot be parsed.

HasClass returns true if the given element has the specified class.

ResolveURL resolves a URL relative to a parsed URL.

SkipTokenContents can be used when encountering a `html.StartTagToken` to read up until the matching `html.EndTagToken`, discarding everything in the middle.

WalkNodesPreOrder calls `walker` on each node in pre-order.