Just a quick note regarding an issue I came across today. HTML allows space separated rel values to indicate link types. I was using this to indicate a URL was both self and canonical, i.e.
<link rel="self canonical" href="https://www.xoogu.com/2013/space-separated-rel-values-html-vs-atom/" />
I was using the same code in both the HTML and ATOM feed versions of my site, but it turns out that ATOM does not allow space separated rel values. So for ATOM you would require two links, like:
<link rel="self" href="https://www.xoogu.com/2013/space-separated-rel-values-html-vs-atom/" /> <link rel="canonical" href="https://www.xoogu.com/2013/space-separated-rel-values-html-vs-atom/" />
Or don’t include a canonical link in your ATOM feed.