To embed a YouTube video, simply right-click on the video and select “copy embed code”:
and then paste into the HTML field:
Currently, mouse handling can cause problems during editing. If in doubt, use the popover/style inspector for styling. There is also no difference between edit mode and presentation mode.
In HTML5, there are some interesting controls that you can use in a prototype. Here e.g. input type="range":
The general styling of HTML elements can be done via the header_incluide.html file.