I opened a related discussion about whitespace on WICG that may be worth a read before any changes are necessarily made regarding this.

the *Class methods have consistently failed to handle "space characters" correctly, and I'd reported a related bug a few years ago.

There are exactly 5 characters that are used to separate classes in HTML/CSS. Those characters in ascending unicode order are:

• tab (\t, U+0009)
• line feed (\n, U+000A)
• form feed (\f, U+000C)
• carriage return (\r, U+000D)
• space ( , U+0020)

When other characters that are matched by the regular expression pattern \s are used in classes, jQuery fails to behave correctly according to the spec.

A quick example of this failure:

$({
  nodeType: 1,
  getAttribute: function () {
    return 'foo'
  },
  setAttribute: function (name, val) {
    console.assert(val === 'foo foo\u00A0bar', 'no-break space should not be treated as a class separator');
    console.log(JSON.stringify(val));
  }
}).addClass('foo\u00A0bar');

which produces an assertion failure and 'foo bar' logged to console.

fiddle

The problem is that classes.js uses rnotwhite which is (/\S+/g) and not an appropriate regular expression to use for splitting strings of classes.