Border Radius Preview Tool
UI

Instead of a number input, a range input could be used - with the minimum value being zero and the maximum value being half of the lesser of the height and width - in this case 88 pixels (i.e. 175px / 2). With such an input there is no need to validate user input to ensure non-numeric characters are entered.

With the current interface, the event handler that fires when a key is pressed does not execute when the value changes from other means (e.g. mouse click). To be thorough, it might be better to observe the change event on the input as well.

Code

The JavaScript code makes good use of const for values that don't get re-assigned, and uses getElementById() (as I mentioned in a review of your Nav scroll code). I like how the constant for the warning time is in all capitals, and all lines are terminated with a semi-colon.

The regular expression could be simplified slightly to use the shorthand character class \d instead of [0-9].