I’ve seen this validation error in Eclipse for a few releases and finally got tired enough of seeing it that I did some research.
The simplest method to silence the warning is to add a DOCTYPE to the build.xml files, between the XML declaration and the project, there are a few more complicated methods, but this one works well for the cases I’ve experienced.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE project>
<project name="Example"...>
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