For Example : <https://hci.uni-wuerzburg.de/>
The Ifram :
{.iframe
width="100%" height="500px" style="border: 2px blue solid;"}
Next slide is showing an Iframe as background
{.iframe}
the PDF : <https://pandoc.org/MANUAL.pdf>
The Iframe :
{width="100%" height="500px"}
Next slide is showing an PDF Iframe as background

markdown source code :
{.iframe .resource style="border: 2px blue solid;" height="500px"}
html source code :
<iframe
data-src="../reload.html?model=../../meshes/blubb.off"
class="iframe
resource">
Browser does not support iframe.
</iframe>