Try out two simpler restatements of the sizes algo.
Capture the fact that only the last <source-size> can be a <length> by itself.
Bah, simple version was missing the requirement that the <media-condition> be true, and fixing that makes it cross my prose-complexity threshold.
Never mind, all the entries can be just a <length>.
Remove the simplest algo again. It really is just too simple.
Make it clearer that starting out empty triggers the step 1 condition too.
Be more explicit about what values are allowed in <source-size-value>.
Merge pull request #153 from ResponsiveImagesCG/syntax-algo

Simplify language of the sizes parsing algo.