When an chromium-internal page, i.e. an error page, is loaded, we see the following warning on the console:

*** /deep/ combinator is deprecated. See https://www.chromestatus.com/features/6750456638341120 for more details.

The reason is that some internal pages still use the deprecated /deep/ CSS feature.

There's an upstream issue: https://bugs.chromium.org/p/chromium/issues/detail?id=498405

In the case of error pages, the use of /deep/ is within chromium/ui/webui/resources/js/i18n_template_no_process.js if someone needs to hack this away...

Alternatively we could suppress this particular warning until the issue is fixed upstream.