A PCB consists of multiple layers, each one of which is described by a gerber file. gerber-to-svg renders each individual layer to svg; this project aggregates all the layers and stacks them up (hence the name) in the proper order.
We use gerber-to-svg (it is the best we've ever worked with) server-side, but we use svg-stacker to generate images for the cases where we need them (thumbnails, printouts, etc.)
On the front-end, we use html5 canvas rather than a pre-stacked SVG.