Wordpress delivers CSS code which is deleted/hidden after the web page has loaded. How does it work?

I have been trying to replicate this land page because I like its design and I want to take it for a test web page. The thing is that I couldn't get the expected design and the trick was in the class name of the gallery, especifically this name: gallery-columns-3

When I inspected the web page and looked for gallery-columns-3, Chrome was saying that it found the name 2 times: once in the class declaration and another one inside an style label.

Here comes the interesting part: I was trying to find that name inside the style label and I double clicked inside the DevTools box to look for it, but I found nothing, and when I clicked on the web page to deselect the DevTools box (doing that I re-executed the CSS code), the web page lost its natural design.

You can replicate it and see how the web page behaves. When the second style label is re-executed, the web page losses its desing and it looks like raw HTML.

I know a little bit of HTML, CSS and PHP, but this behavior doesn't make sense for me.

What is happening behind the scenes? I don't mind if you think explanation is too technical.

source https://stackoverflow.com/questions/76534679/wordpress-delivers-css-code-which-is-deleted-hidden-after-the-web-page-has-loade