Content of beforeShowDay option

In the docs, it's stated that through the use of the beforeShowDay option, it's possible to have custom content in place of the default value of day:

https://github.com/uxsolutions/bootstrap-datepicker/blob/fb8776d65825068b9f914ab37d6fd847c951f488/docs/options.rst?plain=1#L44

Now, what kind of content is accepted there? Is HTML accepted?