Yeah, the CSS OM is really horrible too. CSS Animations is another area where you end up feeding huge generated strings into the DOM -- in theory Web Animations is meant to improve this, though personally I feel like the API too high level and ends up being really large because of this :(.
In your example, I think it'd only be a small patch (for WebKit, where my experience is) to optimize "elem.style.transform = new WebKitCSSMatrix(a,b,c,d)" without intermediate stringification. Mozilla doesn't expose a CSSMatrix type unfortunately. I've done some similar things for other CSS properties in WebKit -- have you considered submitting a patch? I've found the WK guys super receptive to small optimizations which don't change observable behavior (i.e.: you can't tell if the WebKitCSSMatrix was stringified or not currently) like that.
In your example, I think it'd only be a small patch (for WebKit, where my experience is) to optimize "elem.style.transform = new WebKitCSSMatrix(a,b,c,d)" without intermediate stringification. Mozilla doesn't expose a CSSMatrix type unfortunately. I've done some similar things for other CSS properties in WebKit -- have you considered submitting a patch? I've found the WK guys super receptive to small optimizations which don't change observable behavior (i.e.: you can't tell if the WebKitCSSMatrix was stringified or not currently) like that.