Super Large Fonts

I have a yahoo email account that forwards emails to Thunderbird. One person I receive emails from uses Fastmail. Messages from Fastmail often have a portion of the email that has super large fonts. The messages look normal on the yahoo web mail site, but have super big fonts in Thunderbird. If I cut and past the portions with big fonts into my word processor, the fonts look normal as well. I clipped a portion of the html from one of these emails (attached) and you can see from the "font-size" values why the size is so large with values like "8889%" versus "18px". I have no idea where the bogus values are coming from (i.e. Fastmail, yahoo, Thunderbird). But apparently Thunderbird scales the fonts up where the web browser and my word processor seem to ignore the bogus values. Thanks in advance.