How to put a subscript in the slidify title?

Question

I am making slides with slidify. In my title that is to be specified in the index.Rmd YAML header, I need to use a subscript, e.g. H₂0.

• in Rmarkdown, I would do H~2~0. That does not work.
• in Markdown, I would do H<sub>2</sub>O. That does not work
• in mathjax that can be loaded as widget, I would use H\(_2\)O. That works but the mathjax font and fontsize writting is much bigger than the normal title font and fontsize

Is there a clean way to write a subscript in the slidify title, like in latex?