The content: function defaults to using a system defined iFilter for that specific filetype/extension.
This is basically for extracting the text part of the file.
For example a html file iFilter will roughly extract the text you see in a browser or preview pane, without all the tags etc.
(my assumption; didn't test it)
To bypass this behaviour, use the utf8content: function instead. This will treat the content as (UTF-8) plain text instead.
Depending on the encoding of your file, you mnight need to replace utf8content: with one of the following:
ansi-content:
ascii-content:
text-plain-content:
utf16-content:
utf16le-content:
utf16becontent:
Regarding searching tags, Version 1.5.0.1371a (x64) does find them, and I got results from all my files before.
Re regex:content* and regex*:content, the both give the same results, which are the SearchedWord(s) around <a </a> or not (although finding the "SearchedWord(s)") between quotes in "Content with Count" filter does provide more results, I don't know why.