It seems problematic to allow arbitary non-lowercase attribute on HTML nodes. Because the server has no ability to keep them case sensitive (HTML isn't). So they will end up different on the client.

We should probably warn if the attribute is not a known one *and* is not lowercase.
if (debugID != null) {
    
    return getStackAddendumByID(debugID);
  } else {
    
    var stack = ReactDebugCurrentFrame.getStackAddendum();
    return stack != null ? stack : '';
  }
}
if (__DEV__) {
  var warnedProperties = {};
  var hasOwnProperty = Object.prototype.hasOwnProperty;
  var EVENT_NAME_REGEX = /^on[A-Z]/;
  var ARIA_NAME_REGEX = /^aria-/i;
  var possibleStandardNames = require('possibleStandardNames');
  var validateProperty = function(tagName, name, value, debugID) {
    if (hasOwnProperty.call(warnedProperties, name) && warnedProperties[name]) {
      return true;
    }
    if (EventPluginRegistry.registrationNameModules.hasOwnProperty(name)) {
      return true;
    }
    if (
      EventPluginRegistry.plugins.length === 0 &&
      EVENT_NAME_REGEX.test(name)
    ) {
      
      
      return true;
    }