css blocked due to MIME type text/html mismatch X-Content-Type-Options nosniff

Problem/Motivation

Theme fails in Firefox when CSS aggregation off, which is off to diagnose other problems. No CSS on the home page.

The error in Firefox console:
The resource from “https://example.com/themes/danland/css/style.css,qsss2ge+responsive.css,qsss2ge.pagespeed.cc.giK362ygSr.css” was blocked due to MIME type (“text/html”) mismatch (X-Content-Type-Options: nosniff).

There was also the following error which might mean an underlying fault in Drupal:
The resource from “https://example.com/core/themes/stable9/css/system/components/align.module.css,qsss2ge+fieldgroup.module.css,qsss2ge+container-inline.module.css,qsss2ge+clearfix.module.css,qsss2ge+details.module.css,qsss2ge+hidden.module.css,qsss2ge+item-list.module.css,qsss2ge+js.module.css,qsss2ge+nowrap.module.css,qsss2ge+position-container.module.css,qsss2ge+reset-appearance.module.css,qsss2ge+resize.module.css,qsss2ge+system-status-counter.css,qsss2ge+system-status-report-counters.css,qsss2ge+system-status-report-general-info.css,qsss2ge+tablesort.module.css,qsss2ge.pagespeed.cc.ENVLFfGGjb.css” was blocked due to MIME type (“text/html”) mismatch (X-Content-Type-Options: nosniff).

Steps to reproduce

Install danland-8.x-1.2-beta1 in the latest Drupal 10 and switch off CSS aggregation. Access with the latest Firefox. Have not tested other Web browsers. The CSS fails on the home page.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes