How to avoid error messages in Eclipse 2020-03 for GWT ui.xml files (Language Server) - eclipse

Question

In my GWT 2.8 based project Eclipse 2020-03 shows me lots of error message under Markers -> Language Servers
I am sure that these are not errors. For some reason, Eclipse cannot interpret ui.xml files correctly. The application compiles and runs fine:
My eclipse wide settings for XML validation are as follows:
There are not project-specific settings for this
Here is an extract (not the complete source!!!) of one example of a something.ui.xml file. The project contains lots of those files:
<!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:b="urn:import:org.gwtbootstrap3.client.ui"
xmlns:a="urn:import:com.myapp.client.widgets"
ui:generateFormat="com.google.gwt.i18n.rebind.format.PropertiesFormat"
ui:generateKeys="com.google.gwt.i18n.server.keygen.MD5KeyGenerator"
ui:generateLocales="default" >
<ui:with type="com.myapp.resources.text.CommonMessages" field="msgs" />
<ui:with type="com.myapp.resources.text.SummaryMessages" field="sMsgs" />
<b:Modal closable="true" fade="true" dataBackdrop="STATIC" dataKeyboard="true"
b:id="dialog"
ui:field="dialog">
<b:ModalHeader title="{sMsgs.transferOwnership}" />
<b:ModalBody>
<g:HTML><ui:text from="{sMsgs.transferOwnershipDescription}"/></g:HTML>
<b:ListBox
ui:field="userListBox"
multipleSelect="false"
width="100%"
/>
<b:Alert type="DANGER" ui:field="participantWarning">
<b:Icon type="WARNING"/>
<g:HTML><ui:text from="{sMsgs.transferOwnershipNotParticipantWarning}" /></g:HTML>
</b:Alert>
</b:ModalBody>
<b:ModalFooter>
These are the error messages:
Mouseover on the error always shows:
although it is declared above:
[...]
xmlns:b="urn:import:org.gwtbootstrap3.client.ui"
[...]
Import: The application compiles and runs fine. So I just want to suppress those error messages (show in error) as they are not really error messages. :-)
These resources might be helpful:
Why are there multiple Angular language server tabs in my Eclipse console

Answer 1

You can disable this validation by unchecking the box at Eclipse -> Preferences -> Language Servers -> XML.

Answer 2

I had the same problem (Eclipse 2020-06).
Disabling XML Language Server for XML content type worked for me:

Answer 3

By "must be declared", it means declared in a Document Type Definition (DTD) or XML Schema, also known as "grammar".
The line you quoted:
xmlns:b="urn:import:org.gwtbootstrap3.client.ui"
It says there's a namespace "b", but this line has no other meaning. It doesn't, for instance, declare which elements (like Alert) are valid in that namespace.
If you are not worried about validating your XML, and only wish to turn off the error messages, I would suggest you change your options:
No grammar specified: Warning → Ignore
This will stop Eclipse trying to validate the XML against a grammar, but should still show errors for malformed XML (tags closed twice etc.). You can configure this option on a per-project basis, if you prefer.
The alternative would be to provide a DTD or Schema and reference it in the XML. You will probably have to write such a grammar yourself unless gwtbootstrap3 provides one (which I don't think it does).
To clear existing validation errors, you may have to manually delete them.