I've seen this answer but my I can't see what could possibly be the problem in my own code. Here is the my uibinding xml:
<!DOCTYPE ui:UiBinder SYSTEM "http://dl.google.com/gwt/DTD/xhtml.ent">
<ui:UiBinder xmlns:ui='urn:ui:com.google.gwt.uibinder'
xmlns:g='urn:import:com.google.gwt.user.client.ui' xmlns:a='urn:import:com.google.gwt.app.client'>
<ui:style field='iphone' src='Render.css' />
<div id='iHeader'>
<div
style='display:block; opacity:1; -webkit-tranform:translate3d(0px, 0px, 0px);'>
<a>
<span id='waHeadTitle'
style='display:block; -webkit-tranform:translate3d(0px, 0px, 0px);'>
<g:HTML ui:field="title" />
</span>
</a>
</div>
</div>
</ui:UiBinder>
Then I get the error message: Found widget in an HTML context Element <g:HTML ui:field='title'>