svg animations don't work in flexdashboard but they do in Rmd

Hi all,

First and foremost, this is a great package. I extensively use all of rstudio's packages and have a soft spot for each of them. flexdashboard is no exception. I appreciate all of your hard work.

This isn't something that I would expect to be fixed, but I was curious if anyone might know why I can create an svg animation in Rmarkdown, but not flexdashboard?

As an example, if I run this in Rmarkdown, I get the expected output and interactivity on click.
But if I run this in flexdashboard, nothing happens.
Any ideas why?

<svg width="500" height="350">
  <circle id="orange-circle" r="30" cx="50" cy="50" fill="orange" />
  <rect id="blue-rectangle" width="50" height="50" x="25" y="200" fill="#0099cc"></rect>
  <animate 
           xlink:href="#orange-circle"
           attributeName="cx"
           from="50"
           to="450" 
           dur="5s"
           begin="click"
           fill="freeze" 
           id="circ-anim"/>
  <animate 
           xlink:href="#blue-rectangle"
           attributeName="x" 
           from="50"
           to="425" 
           dur="5s"
           begin="circ-anim.begin + 1s"
           fill="freeze" 
           id="rect-anim"/>
</svg>
<p>Click on the circle to animate it, and animate the rectangle after it.</p>

Start with the inline SVG example in the issue and reproduce it in both an R Markdown document and a flexdashboard. Compare the rendered output and click behavior to identify the difference; done means the documented circle and rectangle animations work in flexdashboard as they do in R Markdown.