content-box, padding-box, border-box or margin-box causes the clip-path property to be ignored. I'm not sure if they are relevant for SVG, but I'd imagine they should work for HTML. See the full example list on MDN at https://developer.mozilla.org/en-US/docs/Web/CSS/clip-path#comparison_of_html_and_svg for examples. The cases identified include additional shapes (circles, paths, etc) but it should also work without the additional shapes.