Chrome Version: 71.0.3556.0

What steps will reproduce the problem?
(1) Enable the flag "Save Page as MHTML"
(2) Open a site with styles defined using CSSRuleList, e.g. https://twitter.com/i/flow/signup
(3) Ctrl-S > Save as Webpage, Single File
(4) Open the result

What is the expected result?
Loaded MHTML file should look similar to original

What happens instead?
Styles are dropped
Screenshot: https://imgur.com/a/6MTm9XI

The site's first <style> tag has ".sheet", also found in "document.stylesheets".  Since the tag is not serialized, the styles are dropped.