In the recent review for improving the terminology in the comments for HtmlTree, it was noted that the name HtmlTag was somewhat inappropriate and that TagName would be a better name to use instead.

We should rename HtmlTag to TagName.

In addition, the member fields and accessor methods for the block-type and end-tag do not really belong here and are specific to writing out an HtmlTree. This information is better handled locally in HtmlTree, such that TagName becomes a simple enum with no other loosely-related properties.