Define explicit handling rules for HTML 4 elements

At the moment we unwrap all elements not defined in the HTML5 standard by default. However we cannot tell if an unknown element will be treated as a block or inline element by the browser. Therefore we risk merging two blocks of text that would be rendered as separate visual "paragraphs" into a single visual "paragraph". By explicitly define parsing behaviour for "knowable" pre HTML5 tags we can minimise the risk of treating a "knowable" block level element as inline - see issue

The issue names no files, tests, or entry points. Start by locating the logic that unwraps elements outside HTML5, then review which pre-HTML5 elements are block-level or inline. Done means the knowable legacy elements have explicit parsing behavior so block content is not merged incorrectly.

Written by the indexing model from the issue text.