There has always been code that should abide by the web development style guide[1] in ui/webui/ but hasn't been automatedly checked.
Additionally, code from chrome/browser/ was moved to components/flags_ui, components/version_ui, components/about_ui a while ago to be used on iOS and was previously checked by presubmits but is no longer.
I think we should move chrome/browser/web_dev_style to a more generic location and hopefully makes it easier to check a whole CL's web resources with a single method call.
[1] https://chromium.googlesource.com/chromium/src/+/master/styleguide/web/web.md
Comment 1 by bugdroid1@chromium.org
, May 17 2017