How do services like websvodka.ru work?

Question

Web development

I read an article about them on Habré:
habrahabr.ru/company/embc/blog/249075
How to make a simple textual comparison understandable, but here is how the graphical display of everything that has changed takes place while maintaining the page design as in the screenshot here:

and here:

How can this be implemented?

Answer 1

Page design depends on css.
css depends on html.
To save the page design, you need to save the html.
To save html, you need to build AST of both versions and show changes at the deepest possible level.

Answer 2

With HTML and CSS