For the moment, only quirks-mode is completely supported by GWT. Your mileage may vary in standards mode, but it can usually be made to work with a little fiddling. See the known incompatibilities for details.
The good news is that we are planning on making a concerted effort to support standards-mode in an upcoming release once we have a better handle on all the issues that need to be addressed.
The good news is that we are planning on making a concerted effort to support standards-mode in an upcoming release once we have a better handle on all the issues that need to be addressed.