After the recent addition of custom Web Components, the usual feed validators were a bit less happy about my RSS and Atom feeds.

They always marked my feeds as valid, but usually had some recommendations to improve interoperability with the widest range of feed readers.
In particular having non-HTML5 elements does not help with interoperability. Which makes sense as there is no real place in the XML of the feeds to reference the needed JavaScript for rendering the Web Components.

Besides stripping away the <youtube-vimeo-embed> tags and replacing them with a link to the video, I took this opportunity to cleanup some other 'Altlasten' (legacy tech depts).
A lot of time was spent trying to get my head around various encodings/escapings of special characters. When the blog started in 2002, UTF-8 was not adopted yet and all special characters needed to be written as HTML entities.
And what didn't help is that I somehow had a text-only part in my RSS file which tried to deliver a version of my posts without any HTML (but failed to do so properly as it only had the tags stripped away but did not revert all the HTML character encodings.
Of course the various resulting & characters nicely clash with XML encoding).

There were some other oddities from the past, such as empty post titles and HTML tags inside titles.

I ended up cleaning up most of this and got rid of the text-only representation and corresponding encoding/escaping problems. (using <![CDATA[ and ]]> with the HTML content inside makes life so much easier)

The still remaing recommendations to improve are about relative links and <script> tags.
The relative links are all due to my replacing of dead and non-archived link destinations with a single #. So they are more a 'false postive' than a real problem, the links are dead either way.
The <script> tags are more problematic, as they result from my embedding of GitHub Gists in posts. The code in the Gists is loaded, rendered and nicely highlighted with color by the script, thus not easy to replicate in a feed.

Probably the best approach for the Gists would be to find a way to properly include the content into the posts. So that it is rendered nicely when viewed in a browser, and has a meaningful text-only fallback in the feeds.
Would make sense to self-hosts these anyways. Next rainy weekend project there you are :-)