Since a few days Youtube changed the rendering of fonts especially digits which leads to some strange looking fonts like those: http://imgur.com/a/zIWy9
The issue appears on all browsers as it is linked to the noto font. It tries to use the Arabic part of the noto font to render the digits.
Youtube is aware of that problem already: https://www.reddit.com/r/youtube/comments/643j9e/is_there_a_new_number_font_on_youtube/dfz6a7d/
To fix the issue temporary you can blacklist the using of the arabic noto font (of course don’t do this if you need that specific font).
To do this create a directory ~/.config/fontconfig/conf.d/ and add the file 78-Reject.conf to it with the contents
<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<!-- Generated by Font Manager. Do NOT edit this file. -->
<fontconfig>
<selectfont>
<rejectfont>
<pattern>
<patelt name="family">
<string>Noto Naskh Arabic UI</string>
</patelt>
</pattern>
</rejectfont>
</selectfont>
</fontconfig>Save this file and restart your browser.
This should fix the font rendering issue on Youtube for digits.