There are some HTML tags that are not closed by convention. They are allowed to be closed for backwards compatibility with XHTML, but in practice these days they never have a closing slash.

These tags are the ones I know about: meta br link input

For example, the following code

htmltools::tags$meta(name="foo", content="bar")

Currently generates <meta name="foo" content="bar"/> but it should be <meta name="foo" content="bar">