Hmmm what browser are you using? Looks like sites are reverting to system default fonts for you instead of using the correct styles.
Anyway, the fixed nav bars on sites aren't all coded the same way, so there's no easy way to selectively remove that element on all sites, but what you
can do is use a browser plugin like Evernote Clearly (
https://evernote.com/clearly/) or Readability (
https://www.readability.com/apps) or the native reading view in Safari if you're on OS X to transform pages into a consistent, readable format.
Needless to say, saving them to Pocket/Instapaper/Readability accomplishes the same thing, but then you have to go to their sites or apps to read.