1. apply XSL Transformations to atom feed.
2. include transformed feed to index.html as iframe.

https://set0gut1.github.io/atom-xslt-example/

docs
├── atom.xml    : feed data.
├── wrapper.xml : wrapping of feed, with link to XSL.
├── wrapper.xsl : XSL transformation rules.
├── wrapper.css : style sheet for transformed feed.
├── index.html  : html includes transformed feed.
└── main.css    : style sheet for html.

If I add a <?xml-stylesheet?> declaration to atom.xml, I can see the transformed result in the browser. However, in many cases atom feed is automatically generated and atom.xml is not editable. The wrapper.xml solves this problem, which includes the atom.xml an as external file, and links to the XSLT definition file. I found this technique from StackOverflow: Apply XSL to External XML.