XPath is used in processing XML (JATS and other publishing/standards XML files) and can be used to proces HTML content.

RDF and the related standards are still used in some areas. If the "Batteries Included" standard library ignores these then those standards will need an external library to support them.

Schema.org is used by Google and other search engines to describe content on the page such as breadcrumbs, publications, paywalled content, cinema screenings, etc. If you are generating websites then you need to produce schema.org metadata to improve the SEO.

Did you notice that a new PNG standard was released in 2025 (last year, with a working draft in 2022) adding support for APNG, HDR, and Exif metadata? Yes, it hasn't changed frequently, but it does change. So if you have PNG support in the standard library you need to update it to support those changes.

And if HTML support is optional then you will need an external library to support it. Hence a "Batteries Included" standard library being incomplete.