MustUnderstand Attribute
The @MustUnderstand attribute value containsa whitespace-delimited list of namespace prefixes identifying a set of namespace names. A markup consumer that does not understand these identified namespaces shall not continue to process the markup document, regardless of whether the non-understood namespace was identified as an ignorable namespace on an ancestor element. Markup consumers shall generate an error condition if one or more of these identified namespaces is not understood.
The value of the @MustUnderstand attribute can be an empty or blank string. When a markup consumer encounters such a value, it shall proceed as if the @MustUnderstand attribute was not declared.
The declaration of a Version 2 attribute causes a Version 1 markup consumer to trigger an error when processing the last <Circle> element.
<Circles xmlns="http://schemas.openxmlformats.org/Circles/v1" xmlns:mc="http://schemas.openxmlformats.org/markup- compatibility/2006" xmlns:v2="http://schemas.openxmlformats.org/Circles/v2"> <Circle Center="0,0" Radius="20" Color="Blue" /> <Circle Center="25,0" Radius="20" Color="Black" /> <Circle Center="50,0" Radius="20" Color="Red" /> <Circle Center="13,0" Radius="20" Color="Yellow" /> <Circle Center="38,0" Radius="20" Color="Green" v2:Opacity="0.5" /> </Circles>
Example 10–5. Processing a @MustUnderstand attribute
The value of the @MustUnderstand attribute causes a Version 1 markup consumer to trigger an error when processing the root <Circles> element.
<Circles xmlns="http://schemas.openxmlformats.org/Circles/v1" xmlns:mc=http://schemas.openxmlformats.org/markup- compatibility/2006 xmlns:v2="http://schemas.openxmlformats.org/Circles/v2" mc:MustUnderstand="v2"> <Circle Center="0,0" Radius="20" Color="Blue" /> <Circle Center="25,0" Radius="20" Color="Black" /> <Circle Center="50,0" Radius="20" Color="Red" /> <Circle Center="13,0" Radius="20" Color="Yellow" /> <Circle Center="38,0" Radius="20" Color="Green" v2:Opacity="0.5" /> </Circles>