constraint on schema contains a constraint, expressed in some formal syntax, which cannot be expressed in the structural content model Relationship between scheme attribute and contents Rules in the Schematron 1.* language must be inside a constraint with a value of 'schematron' on the scheme attribute Rules in the ISO Schematron language must be inside a constraint with a value of 'isoschematron' on the scheme attribute supplies the name of the language in which the constraints are defined Schematron ISO Schematron XSLT private constraint language

This constraint uses Schematron to enforce the presence of the spanTo attribute (which comes from an attribute class) on the addSpan element:

Enforce the presence of the spanTo attribute The spanTo= attribute of is required. Implement an accessibility rule which says that pictures should have textual explanations You should provide information in a figure from which we can construct an alt attribute in HTML

This constraint uses a language which is not expressed in XML to check whether the title and author are identical:

(output = leq(title,author) "title and author cannot be the same")