The problem Highlighter Browser Extension solves

A Browser Extension that lets a user highlight text and take notes on a webpage, which are retained on subsequent visits
This extension enables you to highlight text on any web page. Choose from five different colours to highlight text, and the next time you visit the page your highlights will be waiting for you. Also, you can take short notes that are retained on your next visit.

Available on Mozilla Add-Ons Gallery and Chrome Web Store