I prefer it when you put a bit of humour in there
The base64 encoding is being used where mobile phones are the target, trying to squeeze out every last bit of performance.
The problem with adhering to a rigid doctrine is you will end up doing it at the cost of flexibility. Each project is different and has different criteria to fulfill, so you can only use the doctrine as a rule of thumb, and you have to be willing to ditch it when it is appropriate to achieve the desired effect.
There is no one size that fits all, so best to be agnostic and look into many different techniques.
In this instance it is SEO, so title is important, longdesc could be (that made me think). Putting images that are not to be indexed referenced from the CSS is appropriate, but not the doctrine of presentation into the CSS, which is ambiguous in this context, though it would get about 90% of the desired effect.
The doctrine sort of works, but it is not the best thing to do in every scenario, that is the subtelty.
When people say 'everything looks like a nail to a man with a hammer', that's the problem of the doctrine. And, yes that can also be said of using tables over other methods, which is why I don't advocate tables as the best solution for everything either. Different criteria lends itself to different solutions.