Steps to reproduce the issue

1. Go to /administrator/index.php?option=com_plugins
2. Open TinyMCE and click the Advanced tab
3. Set the HTML Width to 50%
4. Click Save

Expected result

If percentages are NOT allowed, I would expect one or more of the following:

• The field indicates that only pixels are supported
• Field shows a validation error if I enter an %
• TinyMCE accepts I've entered a percentage for the width

Actual result

Success alert is shown, but the parameter is blank