Some HTML5 tags are missing

[cpsievert]

May want to consider programmatically constructing classes using https://developer.mozilla.org/en-US/docs/Web/HTML/Element

For reference, this is how the {htmltools} package queries all non-obsolete tags https://github.com/rstudio/htmltools/blob/bc226a3/scripts/generate_known_tags.R#L37

Happy to stab at a PR there's interest in this.