It seems like not long ago in OL2
there was a nice way to style layers with SLD
and it seems like now it is not possible to do the similar thing in
SLD is not yet supported by OpenLayers 3. I am currently working on OpenLayers 3 support for the mapbox-gl-style spec: ol-mapbox-gl-style. It is a small separate library.
There might already be an SLD to mapbox-gl-style converter. If there isn't by the time I'm done, there should be enough funding left to also work on an SLD converter.
Credit goes to Boundless and their customers for funding this effort.