I deleted the closing bracket for the title of the index.html page on my site, then opened it as a file in Firefox-ESR and clicked the "View Page Source" option.
It highlights the beginning of the error in red and the metatag underneath the error is highlighted in red. …
View attachment 11383
I guess, the redness of the latter is because it can not be distinguished from the former. Visualise these:
<title>Building A FreeBSD Desktop From Scratch</title <meta name="viewport" content="width=device-width, user-scalable=yes" />
</title <meta name="viewport" content="width=device-width, user-scalable=yes" />
The normal view-source: view of the non-broken page: