How to test function with browser specific execution?

I have next function:

fillText: function(text) {  
    this.node.innerHTML = text;

    return {
        width: this.node.clientWidth,
        height: this.node.clientHeight
    };
},

And unit test for it:

it('Should return width and height of text node.', function() {
    var sizeInPx = unit.fillText('ABCDEFGHIJKLMNOPQRSTUVWXYZ');

    // width and height of text is different for different browsers
    var widths = [408, 413, 421];
    var heights = [13, 14];

    expect(sizeInPx.width).to.be.oneOf(widths);
    expect(sizeInPx.height).to.be.oneOf(heights);
});

This test is non cross-platform, because widths and heights specified for different browsers (Chrome, IE ...). And at now this test fails on MacOS.

How to test that function right?