10.3. Preinstalled Fonts and User-Installed Fonts
https://drafts.csswg.org/css-fonts-4/#preinstalled-and-user-installed-fonts

User Agents may choose to ignore User-Installed Fonts for the purpose of the Font Matching Algorithm.

!!!! This will create consternation for a great many international users around the world. A huge number of people use languages which are not supported, or are not supported well, by preinstalled fonts. The almost universal approach, currently, for people who use less common or endangered languages on the Web is to find a page where someone has created a font (and often keyboard) that you are expected to download in order to see or create content. (Those fonts are often used by applications other than browsers, too.)

Furthermore, quite often there are also particular font styles that content authors want to see used, often contrastively, and there's a likelihood that a user will have installed additional fonts (such as using a Kano font for Nigerian ajami text, or a Mool font style to distinguish Khmer headings, etc.) in order to make the text readable/locally relevant. For many languages, the system may provide a Noto font, but usually only the one (and generally the Noto fonts don't reflect the design users would want to see for their text, since it focuses on producing Noto-harmonised, large, sans-serif, monoline glyphs).

Of course, this plays into the issues related to privacy, but if the motivation for the sentence is that, it should say so, at least. But that discussion is not yet concluded, so in the meantime the i18n WG would like to see that sentence removed.