Editor for teaching TEI - features

This is an attempt at listing the minimal set of features that a TEI editor used for teaching purposes should have.

Basic required features

 * multiplatform
 * validation with Relax NG
 * contextual suggestions (following the schema)
 * XSLT 2 transformation
 * easy to use
 * free
 * syntax highlighting (?)

Good-to-have but not mandatory

 * xPath query
 * Inline documentation (i.e. the little pop-ups with the definition of the element)
 * pre-set templates (example?)

Features we can live without in a "teaching editor"

 * No XSLT 3 (as a consequence of the fact that no commercial tool such as Saxon PE/EE could be used in such an editor)
 * No need for XSLT or XQuery debuggers
 * no need for database connectivity
 * no need for a built-in SVN (etc.) client
 * no need for a Tree Editor such as the one offered by oXygen
 * no need for Compare Files/Directories tools
 * no need for big-file editor or big-file support in general
 * no need for syntax highlighting and editing support for some file types which are not XML-based (JavaScript, CSS, JSON, etc.).