Regex remove some elements of the style attribute

If this attribute is with the correct syntax, the safest is to interpret what is set and remove it by properties. By regex it seems to me half blind and it can have flaws. One suggestion:

var keepStyles = (function() {
    function getStyles(el) {
        var attr = el.getAttribute('style');
        return attr.split(';').map(function(chunk) {
            var rule = chunk.split(':').map(function(part) {
                return part.trim().toLowerCase();
            });
            return {
                type: rule[0],
                value: rule[1]
            };
        });
    }

    function setStyles(el,styles) {
        var string = styles.reduce(function(str, obj) {
            return str + [obj.type, obj.value].join(':') + ';';
        }, '');
        el.setAttribute('style', string);
    }
    return function(toKeep, el) {
        var current = getStyles(el);
        var keep = current.filter(function(obj) {
            return toKeep.indexOf(obj.type) != -1;
        });
        setStyles(el, keep);
    }
})();

So you generate a function that can receive an array with the styles to maintain ... and use like this:

keepStyles(['border-top'], document.querySelector('td'));

Example: link

You can try removing the other styles using replace, see the example below, in javascript:

style = "color: red; background: #000; position: fixed; top:0; left: 0;";
style.replace(/color:.*?;|top:.*?;|left:.*?;/g,"");

The replace will remove: color, top and left;