Building a website or chrome extension to check for a specific tag in website

Posting here in the hope of getting some advice or be pointed in the right direction. I'm trying to build a website or chrome extension to check for a 'specific tag', 'class' in a website and if that tag exists come back with a result i.e 'you found it'.

I have knowledge of making basic websites and Chrome Extensions. Any tips or advice would be appreciated. Thanks in advance.