Found a one-liner that works in Chrome 66 from: http://www.yournewdesigner.com/css-experiments/javascript-window-close-firefox.html
TLDR: tricks the browser into believing JavaScirpt opened the current tab/window
window.open('', '_parent', '').close();
So for completeness
<input type="button" name="close" value="close" onclick="window.close();">
Though let it also be noted that readers may want to place this into a function that fingerprints which browsers require such trickery, because Firefox 59 doesn't work with the above.