Summary

The normalize_document_structure() method in Dom\Document didn't support content after the HTML end tag. In that case, it failed to remove the end tag, and the subsequent manipulations to normalize further resulted in an overwritten tag that lost all attributes.

This PR changes the regular expressions and the reassembly behavior so that comments are properly kept intact and in the correct ordering across the HTML structure.

It also changes most regular expressions to use atomic groups where needed to avoid any backtracking and improve regex performance.

Fixes #4282

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).