If you’re having trouble with your CSS border not showing up, there are a few things you can check.

First, make sure that the element you’re trying to apply the border to is set to display: block; or display: inline-block;. If the HTML element is set to display: none it’s invisible and thus you won’t be able to see it’s border.

Next, check your CSS code to make sure that you’re using the correct property for the border (border-style, border-width, or border-color) and that it’s applied to the right element. That’s the most common issue as even a single character difference can break your CSS selector.

Finally, make sure that your browser is up-to-date and that you don’t have any browser extensions or plugins interfering with your CSS.