How to change the code in the browser on the fly (when the page is loaded)?

Question

Browsers

In a large VK group, you need to filter out inactive users. I don't trust plugins. they get access to the account of the creator of the group.
You need a plug-in for any browser that allows you to write a simple script and make changes to HTML when loading the page.
The conditions are simple: if such and such a word occurs in an element, say DIV, then we make such and such changes in CSS.
What can be done to accomplish this?

Answer 1

Greasemonkey
Tampermonkey (Chrome)
Tampermonkey (Firefox)

Answer 2

Write user-script for these purposes.
Symphony was right in suggesting browser plugins that will help store, enable, and disable userscripts as needed.