The combination of subresource integrity, cache immutability, and encoding the hash in the URL would be a good solution.  URLs would look like<p><pre><code>  http:&#x2F;&#x2F;www.cloudflare.com&#x2F;cache&#x2F;md5&#x2F;d8e8fca2dc0f896fd7cb4cb0031ba249.css
</code></pre>
and subresource integrity would insure that the value in the URL matches the hash of the content. Files named by their hash can be treated as immutable with confidence.