Spammers sometimes (often enough to bother) are trying to fool spam filters by adding invisible or hard to see text copied from ham messages (e. g. it allows to bypass Bayes). To counter this spamfilters are trying detect and penalize invisible text in HTML emails e. g. <span style="font-size: 0px;">....</span>, display:none, text with the same color as background, e. t. c.

Running something like headless browser (Chrome) to parse HTML emails would require too much resources so spam filters use either simple ad-hock HTML parsers or just regexps. If you use complex HTML markup, e. g. lots of nested div/span tags, different CSS styles on different levels and global <style> block on top of all, they can be confused and will detect invisible text in a message where all text is visible (when CSS/HTML is rendered in Google Chrome; who knows how it would look in different mail clients).

If you use clean and simple HTML, there will be no leftovers like <span style="font-size: 0px;"> and you will not confuse spam filters.