The accent-color CSS property allows the page to specify a color used by form controls instead of using the builtin default color.
For example, "accent-color: red" on an <input type=checkbox> would change the blue color around the checkmark to red.
A number of developers have indicated that they would prefer to be able to change the color of form controls. For example: https://bugs.chromium.org/p/chromium/issues/detail?id=1091531
If this is not implemented by other browsers, then form controls will still work the same, they just won't have the custom accent-color.