One thing to keep in mind is that JSX isn't HTML—it just looks like it. This is important because the JSX transpiler (usually Babel these days) takes the JSX syntax and modifies it from this:
<div className="container">
<p>Testing!</p>
</div>
to something like this:
React.createElement("div", { className: "container" },
React.createElement("p", null, "Testing!")
)
By abstracting over React.createElement calls, you can end up with a project like r-dom, which does the same thing but with a nicer syntax:
r.div({className: 'container'}, [
r.p('Testing!'),
])
or react-hyperscript, which allows an alternative syntax for some properties:
h('div.container', [
h('p', 'Testing!')
])
However, since JSX turns into plain JS calls, any language or syntax that can be converted into React.createElement calls will work great with React, as long as you set up your Rails asset pipeline so that it correctly does the transpilation as part of serving assets.
There is a project that does this with Jade's syntax called react-jade; there are a few differences from regular Jade and some unsupported features, but it may do what you want it to do. In a Rails project, you'd need to find or create a preprocessor that turns the appropriate Jade code into React-specific JS.
There's one other thing I wanted to mention based on a comment in your question:
now I'm writing bloated HTML again inside my components
If you're just talking about HTML's syntax then no problem, but if you're finding that the render methods of your React components are getting large and hard to manage, then it's probably a good sign that you need to break your component into smaller components (see "Tip 4: Embrace composition!" at this link).
