When I open a webpage, the page jumps around on the screen.

Does this happen with another browser?

With all webpages?

Try booting to Safe Mode. That boot up takes minutes and clears caches and other things that may resolve your problem.

How to use safe mode on your Mac - Apple Support

A lot of times just boot to Safe Mode followed by normal boot resolves problems.

from what you are describing, is it possible that you've "discovered" smart zoom? does the page appear to be zooming in and out? you can inadvertently activate smart zoom when handling the mouse. you can go into System Preferences > Mouse in the Point & Click tab and remove the checkmark from Smart zoom.

if Smart zoom was selected, it can cause the symptoms you are describing. but if it is smart zoom, you may want to try and get used to how it functions. i don't know how i used to live without it.

Runningsquirrels wrote:

When I open a webpage the page jumps around on the screen, and the fonts shrink and expand without me touching anything. What should I do?

[Re-Titled by Moderator]

Have you tried rebooting your machine?

Log out, sleep, wake, restart or shut down your Mac

Shut down or restart your Mac - Apple Support

You do not say what exact Mac this is(?)

what macOS(?)

Is this an old issue, new issue, what changed(?)

one particular website you can post the link(?)

or

all websites all browser...