Nested UI elements with attributes

[zbraniecki]

A quite convoluted example I asked WG to evaluate against our model proposals as part of evaluation whether we would benefit from separate Element entities from Placeholders:

key = This is a message with <img/> that is to be placed by a localizer and also <p l10n-id="key2" tooltip="This section is for your viewing only"></p> after it.
key2 = This is a nested <img/> message with a picture.

key = This is a message with <img/> that is to be placed by a localizer and also <p l10n-id="key2" tooltip={key3}></p> after it.
key2 = This is a nested <img/> message with a picture.
key3 = This section is for your viewing only