Most UI testing tools for web applications (like Selenium and Cypress) try to verify that an element is visible to the user by searching the underlying code of the page for a particular locator tag. If the test finds the element locator, the test will pass. However, there are reasons why the element locator can be present but the element won’t appear correctly to the end user:
A bug could render the element off of the end user’s screen.
A different element (like a popup) could be covering the element you’re testing for.
The size of the element could be extremely small or extremely large.