This might seem like an innocent change but it really is another HTML/CSS browser hell hole. If I remember correctly it was Jeff Atwood or jzy who taught me this lesson at a time when Stackoverflow was just out of Beta:

If the keyword monospace is somewhere in your font stack some browsers use 13px as default font size instead of the usual 16px. The workaround used to be to use both, serif and monospace, in your font stack. This used to work some time ago, I don't know if it still does with contemporary browsers.