Text content should be converted to PSD Text Element. Some of CSS effects are better represented using PSD Layer Style effects. Check out the complete PSD file format here: http://www.adobe.com/devnet-apps/photoshop/fileformatashtml/...

I haven't really try LZW compression for the image data, but it seems that generated PSD file for a single webpage is extremely huge (multi megabytes PSD file). Standard Photoshop App generate image data with mostly Packbit compression. There is probably room to improve on that.