CSS normalization

We have a short reset file as part of our stylesheet, but it's fairly minimal and leaves untouched many differences between browsers. This came up recently with Firefox-specific button styles that we had to disable in #8217

We should use something like `normalize.css` to reduce differences between browsers, as it's really labour-intensive to check each style change across all browsers.

Start by locating the stylesheet's short reset file and review the Firefox-specific button-style issue in #8217. Compare the current reset with normalize.css and determine how it should be integrated into the extension. Done means browser differences are reduced without reintroducing the button styling problem.

Written by the indexing model from the issue text.