I noticed that the tag "<content>" used in conjunction with pageProperty taglib is marked as "unknown" in STS (Eclipse) .
This tag is taken up by decorator (SiteMesh) for dump/merge content pieces to layout (it is very useful in many cases).
Let me clarify that although unknown in the STS, it works well !
I don't know if this should be corrected within Grails or STS.
I propose to make this slight correction in next milestone.