Programs / plugins / scripts that get the desired css from the HTML DOM tree for the desired element and nested in it?

Question

PHP

First of all, I appeal to the moderators: by specifying the tags: PHP, NodeJS, I did not spam, but expanded the question to web users who may know the answer to the question and who may be interested in the topic (although I understand that it’s not necessary to do this with tags), especially NodeJs on which some preprocessors are written.
On the question, maybe someone knows programs with which you can quickly get css for the desired element and nested in it.
The essence of the situations in which the program can be applied: for example, there are 2 sites. One is old and the other is a new version. CSS and HTML are different respectively. Sometimes the task arises that while the second site is under development, it is necessary to place some kind of separate page on the first one, in order, for example, to launch advertising on it. The task is to combine css code. That is, the header and footer remain from the old site, and for example, the content block from the new one. Here's how quickly you can get css for the desired block? I'm not saying that everything is correct in order to work out, just to optimize a significant part of the work, it is clear that there may be minor edits.

Answer 1

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