Disable all styles in a @supports rule.

disable_support.js

function disable_support(supports_condition) {
	for (var si = 0; si < document.styleSheets.length; si++) {
		window.console.log("Looking in " + document.styleSheets[si].href);
		var rules = document.styleSheets[si].cssRules;
		for(var i = 0; i < rules.length; i++) {
			if (rules[i].type === 12 && rules[i].conditionText.match(supports_condition)) {
				window.console.log("Removing style index " + i);
				document.styleSheets[0].deleteRule(i);
			}
		}
	}
}

// To disable rules inside: @supports (display:grid) {
function disable_grid_support() {
	disable_support("(\s*display\s*:\s*grid\s*)");
}

// To disable rules inside: @supports (display:flex) {
function disable_flex_support() {
	disable_support("(\s*display\s*:\s*flex\s*)");
}