Schepp
Duplicate data-uris bloat the CSS
If the CSS contains multiple references to the same image, CSS-JS-Booster currently embeds the image for each occurance. This can easily lead to a pretty large CSS file (or many chunks of it).
MHTML does not fall for this. but with data-uris I don't see a viable solution to reducing the overhead. The only way out is to skip images with multiple references. If the WebDev still wishes to make use of data-uris, he must modify his CSS accordingly.
When I have the time, I will do some experiments and tests trying to shift all images to a second lazy-loaded file. But its complicated and also bears the risk to break the CSS-cascade if done wrong. So I can't promise anything and I also don't know when this will be ready (or dismissed).
You should have a "background definition" css file when developing.
It's easier to maintain especially when using css sprites.
True. Did you know that you can already do the lazyloading-thing in Booster? https://github.com/Schepp/CSS-JS-Booster/wiki/Advanced-Usage
$booster->css_lazyload = TRUE;