Modal pop-up HTML code works on other sites or HTML viewers but not in the custom HTML block within a wordpress page?

There are a couple of issues with what you are trying to do. See this for reference: WordPress support for Custom HTML blocks First, you cannot reliably add JS code directly into any block, which is intended only for HTML and text. While the script tags may technically be HTML, your JS code certainly is … Read more

Editing size of h3 tag in wordpress on a one-off basis

If you’d like to use <h3> tags that are the same size as your <h4> tags only on one page, add a specific class to each of the <h3> tags you’d like that size. In the Classic Editor, you should have a “Text” tab where you can input custom HTML. In there, type your <h3> … Read more