How do I add a piece of my HTML to the site?

Question

HTML

You need an extension like stylish (allows you to change the CSS of the site), which adds HTML code to the site.
Well, for example, add additional menu items to VK.

Answer 1

This is done through JavaScript. Here are the addons for Firefox and Chrome:
https://addons.mozilla.org/en/firefox/addon/grease...
https://chrome.google.com/webstore/detail/tampermo...