Facebook has been doing a lot of work in this area, initially implemented in PHP by Marcel Laverdet in 2010, called XHP[1]. Not only does it permit you to embed conformant HTML into your PHP code without inadvertently introducing XSS vectors, but it also allows you to compose HTML primitives into high-level markup. Stefan Parker has been regularly blogging since 2011 about how Facebook uses XHP in its UI design; one of the best posts on the topic is here[2]. You may also recall the now-defunct "Facebook Lite" which was written completely in XHP.

Laverdet also implemented something similar for node.js called js-xml-literals[3], which I don't think has gained much traction. As for how it's different from E4X, refer to this page[4], which makes the argument that parsing and producing XML are two different tasks; E4X attempts to do both while all of these other approaches focus on producing XML.

(I don't think that this perspective on the history of react.js/jsx has been well-publicized before, hence the post.)

[1]: https://www.facebook.com/notes/facebook-engineering/xhp-a-ne...

[2]: http://codebeforethehorse.tumblr.com/post/52824249342/buildi...

[3]: https://github.com/laverdet/js-xml-literal

[4]: https://github.com/laverdet/js-xml-literal/wiki/Differences-...