You should leave the “content” of a page to the actual content, that is text and images. You shouldn’t be pasting custom html, as it is very easy to break and hard to maintain. If you have custom html for a page and don’t care about editing it yourself directly, you should be using custom page templates.
You can learn about them here:
https://developer.wordpress.org/themes/template-files-section/page-template-files/