There were other seamless issues, but I don't believe this is related, so apologies if this turns out to be a duplicate. First noticed on Chrome, reproduced on Webkit Nightly from October 12th. Steps to reproduce the problem: 1. I've attached 2 files: test.html and iframe.html. 2. Download them to the same folder and open test.html locally in your browser 3. You'll notice that the HELLO WORLD text isn't red. 4. Edit text.html to uncomment the iframe tag that has the src="iframe.html" attribute set. Reload test.html, and now you'll see that the text is red. What is the expected behavior? What went wrong? Parent styles in a seamless iframe are being applied correctly only when the iframe content is loaded using a src="" attribute. When the content is loaded dynamically with JavaScript, the parent styles aren't applied, even though it's still a seamless frame. My understanding of the spec is that parent styles should be applied to the seamless frame regardless of where the content comes from, but I might be misunderstanding something.