The new "heights" attribute is part of the layout=responsive ecosystem. It's developed in #1331

The rules are very similar to sizes attribute:

1. If layout is not specified and heights attribute is specified - implied layout is responsive
2. heights attribute can only be specified for elements with layout=responsive - input or implied
3. The format of the heights attribute is the same as sizes with a single exception that % are allowed as values (not sure we actually validate content of sizes attribute).