Boolean DOM properties coerce empty string to false, contrary to HTML standard

<!-- Note: if the issue is about documentation or the website, please file it at: https://github.com/reactjs/reactjs.org/issues/new --> This is in kind of the same space as https://github.com/facebook/react/pull/13372 and is an offshoot of my attempt to better [model React DOM props in Flow](https://github.com/facebook/flow/pull/6727). **tl;dr:** Should React warn when the value `""` is passed into a known boolean DOM prop? --- **Do you want to request a *feature* or report a *bug*?** Depends on interpretation 😅 This is possibly a bug, definitely an inconsistency worth mitigating IMHO. **What is the current behavior?** React normalises values supplied to [known DOM boolean props](https://github.com/facebook/react/blob/69e2a0d732e1ca74f6dc5df9d0ddd0bf24373965/packages/react-dom/src/shared/DOMProperty.js#L278-L331) (e.g. `readOnly`) such that passing the empty string `""` (being falsy in JavaScript) results in the corresponding attribute being omitted from the HTML output. However, in [HTML](https://html.spec.whatwg.org/multipage/common-microsyntaxes.html#boolean-attribute), the empty string is a truthy value in this context; it's one of the values that the standard specifically allows in boolean attributes. The above is a potential source of confusion in itself, but React 16's handling of unknown attributes gives rise to the following hypothetical scenario: a new DOM boolean attribute `foobar` is introduced, some people write JSX code that uses it as `foobar=""` (passed through to HTML, truthy), and later React adds `foobar` to its internal whitelist in a minor/patch version and starts processing it as a boolean (JS falsy, omitted from HTML); this would _technically_ be a breaking change for those people. **If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem.** https://codesandbox.io/s/y0pmz9149x **What is the expected behavior?** There is definitely a clash of expectations here at the interface …