They might be modified in any way, or even removed, in any major release. You should not use beta features if your code is a library itself (that is, if your code is used on the CLASSPATH of users ...

HTML, XML, CSS, JavaScript, Makefile, and Rust. It works passably on Ruby, PHP, VB, and Awk and a decent subset of Perl and Ruby, but because of commenting conventions, doesn't work on Smalltalk, ...