UserAgent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:51.0) Gecko/20100101 Firefox/51.0

Steps to reproduce the problem:
To reproduce bug, create a document containing the following script:

    <script>
    var link = document.createElement('link');
    link.setAttribute('rel', 'alternate stylesheet');
    link.setAttribute('href', 'data:text/css,');
    document.body.appendChild(link);
    console.log(link.sheet);
    </script>

What is the expected behavior?
The `null` value is printed to the console

What went wrong?
A `CSSStyleSheet` instance is printed to the console

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 55.0.2883.87 (Developer Build) Built on Ubuntu , running on Ubuntu 16.04 (64-bit)  Channel: n/a
OS Version: Ubuntu 16.04
Flash Version: Shockwave Flash 24.0 r0

Note that the use of a data URI is for conciseness only. This behavior is also
observable with resources specified via external URLs, however the property is
not defined until after the resource has been loaded.

The Web Platform Tests project includes a test for the interaction of
`alternate` and `sheet` [1], but because this erroneous behavior is not
observable until after the resource has been loaded, the test results in
Chromium are unstable. The test was disabled in the Chromium infrastructure via
[2]. A test modification that accounts for this condition (making the test
consistently fail in Chromium) is currently under review [3].

[1] https://github.com/w3c/web-platform-tests/blob/2b50f8c8d48379a80b1667adc0826f2035dc9440/html/semantics/document-metadata/styling/LinkStyle.html
[2] https://src.chromium.org/viewvc/blink?view=revision&revision=196645
[2] https://github.com/w3c/web-platform-tests/pull/4828