Working on Open Office XML and html these days, I ended up reading and implementing section 17.18.78 of the ISO/IEC 29500 spec. It's the one dedicated to shading patterns. In words we're used to, predefined background images serving as pattern masks. It's not a too long list but the PNG or data URLs were not available as public resource, and I found that rather painful. I am then making my own implementation, in JavaScript, of ST_Shd public. Feel free to use it under MPL 2.0 if you need it.