For wikitext-to-html conversion, we want to have the original html rendering as context. If client may send a render ID (wrapped in an etag) to reference a stashed rendering. If that rendering is not found in the stash (because it has expired or has been evicted), we should check the rendering we have in the parser cache, and use if it the render ID matches.
On wikis with low edit frequency, chances are good that this will work.