Let me see is a web app that you can run locally (or self hosted somewhere) and use to test how some webpage looks like. Note that you need permission to access that page. It is simillar to other testing, scrapping, crawler tools, but I wanted to build one myself.
It is API based service.
Results can be returned using websockets: https://blog.stanko.io/do-you-really-need-websockets-343aed40aa9b
server sent dom events
In python https://scrapy.org/ https://awesome-ruby.com/#-web-crawling