What browser are you using ... Safari?
Somehow you replaced your Arial Bold with "bunnywunny". WTF were you thinking? You now need to figure out how to reinstall the Arial Bold system font somehow without needing to do an OS reinstall.
Before you do anything, look in ~/Library/Fonts/ and delete that font if you see it there, then reboot.
I’m surprised that this system font was not protected by SIP from this human error.
Arial actually is not a system typeface on OS X. Preinstalled, but not required.
Partly true.
But if you mistakenly replace Arial Bold with bunnywunny as I did, then any web browser, whether that’s Safari, Chrome, Firefox, etc. are affected by it. So it is part of the system, because all the bolded content in Google search results, for example, display as bunnywunny, which renders it virtually illegible.
Of course they are affected by it, because Arial is commonly used on the web. When your browser can find the typeface that the website requests then it will show it. You would have exactly the same problem in every other application (e.g. a Word document). However, you can in fact delete Arial without any problems, because browsers and applications always fall back unto the system typefaces. Arial is optional, but that does not mean that it could not cause problems of course. I am not sure how you even managed to get Font Book to identify this typeface as Arial. The font names are usually hard-coded into the font files.
Font Book has a restoration function by the way. It would not have restored Arial, but it would have kicked out this “Bunnywunny” typeface. Google would show Safari’s or Chrome’s default sans-serif typeface, which is probably Helvetica.