As a workaround, you might think of using iframes and adding style attributes to a tag.
So you go to ‘Add New Post’, then pick the ‘HTML’ tab. In this post I added an iframe between 2 thick horizontal lines, which follow right here:
When you click the ‘Visual’ tab you get a large box representing the iframe and where iframe’s contents will be, and the horizontal lines are thick. To see this you will have to eidt this page yourself.
But in Preview mode, or after publishing the page, the iframe has been converted to a link, and the horizontal rules are the default ones.
So WP uses a standard Visual+HTML editor and only does its filtering of your content when you go to publish or preview the page.