Edit in JSFiddle

<div id="demo""></div>

body {
  font-family: 'Helvetica Neue', Arial, sans-serif;
}

#demo {
  width: 100px;
  border-style: solid;
  border-color: red;
  border-bottom-width: 9999px;
}

var demo = document.getElementById('demo'),
    borderWidth = getComputedStyle( demo ).borderBottomWidth,
    output = document.createElement('p');

output.textContent = 'border-bottom-width is set as 9999px but is rendered as ' + borderWidth;

document.body.insertBefore( output, demo );

console.log( demo.style );