It would likely be difficult to do with the relative positioning of the decoration around the main chart and the other calculations that must be done that are relative to the actual width and height. Plus, any time the html5 tag changes it’s size I believe it would need to re-draw everything from scratch again.
The best you could probably do would be to do a two pass rendering for that part of the portlet. In other words, in the initial rendering, the chart is not displayed, but you have some javascript that calculates how wide to make the chart. Then the value calculated by the javascript is posted back to the to the server and it renders the chart with that width on the second pass.
If you think the control should handle percentage widths/height for you, then may want to make your interest in that area known with a feature request in brainstorm @ https://empower.softwareag.com/Products/FeatureRequestsInBrainstorm/default.asp so that request gets on the roadmap.