Offline linker performs the task of generating a valid cache manifest file
when you compile your GWT application.
Static resources that are needed (outside of the compile unit) require
specific inclusion. These files would typically be index.html, css files or
any resources not included within the GWT application. These files are
included through the cachemanifest_static_files property added to your
module.gwt.xml file. The path is relative to manifest, so include a full path
if you include resources outside of the apps path.
<extend-configuration-property name="cachemanifest_static_files" value="/index.html" />
To activate the linker, the following configuration is included in your GWT
module definition (module.gwt.xml file) as follows:
<inherits name='org.gwtbootstrap3.extras.cachemanifest.Offline'/>
<add-linker name="offline" />
Finally, include the cache manifest file within the html page that loads your
GWT application, as follows:
<!doctype html>
<html manifest="<modulename>/appcache.manifest">
....
</html>