TypeScript: check for CSS if() support

supportsCssIf.ts

const CSS_NO_IF_SUPPORT = /body\s*\{\s*\}/;

/**
 * Checks if the browser supports CSS if().
 * @returns true if the browser supports CSS if(), false otherwise
 */
export function supportsCssIf(): boolean {
    if (typeof document === "undefined") {
        return false;
    }

    const style = document.createElement("style");
    style.textContent = `
        body {
            background: if(media(screen): red);
        }
    `;
    document.head.append(style);
    /*
     * In browsers that support if(), cssText will include the background property with the if statement,
     * else, it will just be `body { }`.
     */
    const matchesEmpty = CSS_NO_IF_SUPPORT.test(style.sheet?.cssRules[0].cssText ?? "");
    style.remove();
    return !matchesEmpty;
}