Perhaps someone else has encountered this:

Creating a CTA button using the padding/borders technique, on Litmus testing, I noticed that the CSS on the <a> tag was not rendering at all in or Office 365. It took me a while, but eventually figured out that it was the URL - I was using '#' as a placeholder. It seems that any kind of non-standard URL not only disables the CSS on the anchor, but breaks all following page content. For example using <a href="MERGETAG"> for Mailchimp.