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This issue affects

• The site generated by GitHub Pages
• Building sites locally

What did you do (e.g., steps to reproduce)

Added https://github.com/primer/jekyll-octicons to site while developing locally (thanks @jonrohan!)

What did you expect to happen?

Lovely GitHub svg icons, space magic, etc.

What happened instead?

This error in [repo]/Settings -> GitHub Pages:

Your site is having problems building: The tag octicon on line 3 in /_layouts/test.html is not a recognized Liquid tag. For more information, see https://help.github.com/articles/page-build-failed-unknown-tag-error.

I've since discovered that only plugins found in this project are supported by GitHub pages (which makes sense!)
I'm about to fix this by manually digging out the assets I need, but I figure primer/jekyll-octicons would be a good plugin to support here, given the context :)