Percent units work in the topic preview while writing a page but don't work in actual topics. Percent units aren't necessary, though having the preview show what the topic actually looks like would be nice.
Repro
Make a HTML img like this in a topic:
This image takes up 80% of the parent element width in the preview. However, once posted, the image’s width is 80 pixels, instead of 80% of the parent element’s width.
Example
The preview of this article:
It looks like the system switched the image to markup: <img src="" width="80%"> → 
I haven’t ever seen Dev Forums do this. It also changed the image size to the actual image resolution (which caps when it gets larger than the parent element).
