Dropped
XSLT templating leaves along with SharePoint. I won't miss it: recursion instead of loops and constants posing as variables never did grow on me.
Using
Before the REST API showed up in SharePoint, the main way to build the interface was the combo of a web part that queried data and emitted it as XML, plus an XSLT template to transform it into HTML.
The language is extremely unintuitive. All variables are constants. Any loop-like logic has to be implemented recursively.