CudaText editor is free, open source. It has code-tree for lot of "lexers", also for HTML and XML. Example look with short HTML:
In the plugins (Plugins / Addon Manager / Install) you will find "treehelper: HTML_XML" which is more advanced version of HTML code-tree. More tags and more detailed look.
I am reminded of Kompozer. It is free, and open source, although it doesn't look like it has been updated for a few years.
This basic HTML:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<meta content="text/html; charset=ISO-8859-1"