The issue with formatting is happening on the Discourse end. It is stripping out the table and iframe tags from the WordPress post. You can see how it works by creating a topic through the Discourse composer with table and iframe tags. The tags and the content inside of them will be removed.
It looks like you are using an iframe for displaying Youtube videos on your WordPress site. You can get videos to display on both WordPress and Discourse by just adding a link to the video on it’s own line in your WordPress post. If the video link is the first item in the post, you need to add a blank line above it.
This is a post with a Youtube video.
https://www.youtube.com/watch?v=sg209CadVQM&list=PL41PZuZMej3etzs5aj6R0EQ5S528otfec
I don’t see a good simple solution for displaying a table from a WordPress post in a Discourse topic. I’ll look at it a bit more and see if I can come up with something.
The problem with this is the same as with iframe and table tags. If a class is added on the WordPress end it is stripped out when the post is parsed by Discourse. Discourse styles the small tag with font-size: 9px. To my eyes this is too small for any text. You can add a custom style to your forum to change that. If you only want it to effect small tags in posts, .cooked small { font-size: 80%; } should work.