const (
    // ErrorToken means that an error occurred during tokenization.
    ErrorToken TokenType = iota
    // TextToken means a text node.
    TextToken
    // A StartTagToken looks like <a>.
    StartTagToken
    // An EndTagToken looks like </a>.
    EndTagToken
    // A SelfClosingTagToken tag looks like <br/>.
    SelfClosingTagToken
    // A CommentToken looks like <!--x-->.
    CommentToken
    // A DoctypeToken looks like <!DOCTYPE x>
    DoctypeToken
)