As someone who has worked on web development, desktop development (in the good old days of Delphi) and used Flex, I can categorically say that any HTML-based web development is painful and slow compared to using Flex or any homogenised development system.
HTML+JavaScript+AJAX+CSS+WS+Java+Spring Web (or Tapestry or Struts or....)
OR
Flex (CSS+MXML+ActionScript) + Java on the server
OR
JavaFX (CSS+JavaFX Script) + Java on the server
I am personally fed up with trying to use HTML as a platform for applications. It isn't fit for purpose and it was never intended to be. It takes good old fashioned ideas of sane development and uses them as to clean its rear end.
NEWS: INTERNET BROWSERS ARE A RUBBISH PLATFORM
Your computer account is overdrawn. Please see Big Brother.