What is wrong with the tag

Three things. It is non-conforming HTML, so validators flag it and nothing guarantees future support. It has no pause control, so a visitor who finds moving text hard to read cannot do anything about it. And it has no sense of the page around it: the speed is a loose scale of its own, not a distance over time.

Find where it is

If a theme or an old post still uses it, search your content and your theme for <marquee. In a block-editor post it usually sits inside a Custom HTML block. In a classic theme it may be in a template file, where changing it means editing the theme or its child theme.

Replace it with a block

  1. Add a Text ticker where the old tag was.
  2. Move the old text into paragraphs inside it, one per message.
  3. Set the speed. Instead of the tag's loose scale, you choose pixels per second, so you can pick a pace and know it will be the same on every screen.
  4. Delete the Custom HTML block.

Try the replacement on your own text here:

Try a marquee

Change the settings and watch the same engine the plugin uses. Nothing you type leaves this page.

What you gain

A Pause button that stays beside the strip. Pausing on hover and on keyboard focus. A loop that has no empty stretch after the last item. An accessible name for the region. And a strip that stands still, and can be scrolled by hand, for a visitor whose device asks for less motion.

Whether a pause control is a requirement for your site is covered in the WCAG 2.2.2 guide.