Good work. Can you back port this change?
Done some performance testing today and noticed that when there is a large hierarchy of controls on page, performance degrade quite a bit.
I've tracked down the problem to AbstractContainer#getHtmlImports which creates a large buffer for HTML imports. Each container in the hierarchy will create this buffer, whether its used or not, which is wasted memory and causes the performance degrade.
I've checked in a change to create a buffer of size 0 which resolves the above issues. And since subclasses such as Form creates their own buffers this should not impact existing code.