User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_2) AppleWebKit/604.4.7 (KHTML, like Gecko) Version/11.0.2 Safari/604.4.7 Steps to reproduce: Load https://puppet.com/docs/puppet/5.3/type.html and start a timer. It's a very long autogenerated page with many elements. Actual results: The page takes approximately 5 minutes to load. During the load it pegs CPU at 99%+, and then after load takes about 500+MB of RAM for this single page. This occurs on Firefox 58.0b6 on Linux and 57.0 on MacOS. It does not occur on 56.0. Expected results: The page should have loaded appropriately. The page works fine on Safari, Chrome, IE, FireFox versions prior to 57.0. Our local testing indicates that it may be an issue with the ModernizrTests Drupal module, so you might start seeing other Drupal sites reporting issues. Commenting out the return line of the csscalc test seems to mitigate this issue.

Disabling stylo seems to fix the problem. Profile with stylo enabled: https://perfht.ml/2AbXbWJ Profile with stylo disabled: https://perfht.ml/2Abp5lR Emilio, can you take a look?

Yup, see bug 1420741 for a diagnostic. I'll work on a proper fix tomorrow. It is the same page so marking as a dupe.