DOM attribute dateTime isn't lowercased when rendered to string

[larsnystrom]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
The DOM property dateTime isn't converted to its lowercase attribute counterpart datetime for the <time> element when the element is rendered with ReactDOMServer.renderToString().

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://jsfiddle.net or similar (template for React 16: https://jsfiddle.net/Luktwrdm/, template for React 15: https://jsfiddle.net/hmbg7e9w/).

const React = require('react');
const ReactDOMServer = require('react-dom/server');

const markup = ReactDOMServer.renderToString(React.createElement('time', {
	dateTime: "2017-11-02T20:49:25+01:00"
}));
console.log(markup);

The above will print <time dateTime="2017-11-02T20:49:25+01:00" data-reactroot=""></time>.

What is the expected behavior?

The above output should be <time datetime="2017-11-02T20:49:25+01:00" data-reactroot=""></time>.

Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?

React 16.0.0
React DOM 16.0.0
Node v6.11.5

[gaearon]

This is a duplicate of #10863.

We’re hesitant about this. HTML is case-insensitive so technically our behavior is correct even if a little surprising. There's absolutely nothing wrong with emitting attributes with any casing, even DaTeTiMe would be perfectly understood by browsers, and the difference would be completely unobservable for the client-side code.

Though we potentially can “fix” this to be less surprising (#11110) we’re not confident it won’t lead to a slight performance regression. So it might not be worth it. We’d need data from people using SSR in production to tell if this would be a problem or not.