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Some content elements can have fallback content: content that is to be used when the external resource cannot be used (e.g. because it is of an unsupported format).
The element definitions state what the fallback is, if any.
Some content elements can have fallback content: content that is to be used when the external resource cannot be used (e.g. because it is of an unsupported format).
The element definitions state what the fallback is, if any.