Background Information
Just to get some context for my question, I started at a new SME in the UK earlier in the year. I was employed as a PHP developer, but I am Scrum certified and have worked for large and small companies; so alongside moving the company to Agile processes, I've been asked to propose and head QA, automation, testing and releases. Outside of web development, I have no first-hand experience with any of those processes.
My employer currently has a lot of very basic HTML and CSS websites with some JavaScript and not very much PHP. It's very important, for SEO purposes, that any changes to a website does not affect, auto-generate or in any other way mess with the HTML pages served. Every site that has been migrated into any framework has lost SEO power and lost leads and therefore lost revenue. So the development time has been costly for no return. In fact, our Jekyll websites are going to be decommissioned and we're going to redeploy the old flat and non-responsive HTML versions of the sites.
Because of this, a bespoke and very light weight request/response PHP framework has been developed in house. The purpose is to migrate the old HTML websites to run in this new framework, with the obvious advantage that pages can be partially templated etc.
What I need to know
I need a way to prove that the HTML served by our framework matches the corresponding HTML source on the live website, and where the differences are (if any exist). We can do this manually (but that's going to take time away from development one way or another).
Is there an automation tool that will compare one webpage (example dev page at https://test.example.com/my-page.html) against another (example corresponding live page at https://example.com/my-page.html) and detail where the HTML source is different? I'm sure that I can write a PHP script which will do this, but if there's something out of the box that'll do the same job then that's saved [expensive] dev time.
<img src="https://test.example.com/images/main.jpg" />
vs<img src="https://example.com/images/main.jpg" />