Update: GitHub now has a tab size setting: https://github.blog/changelog/2021-09-21-tab-size-rendering-preference/
By default, in browsers, the tab character takes up 8 spaces in width. This makes tab-indented code hard to read. GitHub could have easily overridden this with a more reasonable number like 4, but they have chosen not to. Hence why this extension exists. It sets the tab width to 4.