TagIndex is not effective in apps with changing semantics. The ability to affix more specific traversal cascades with psuedo selectors would improve performance and specificity in targeting items in the DOM tree.

Examples
TagSemanticPath: 'is exactly' 'my-custom-element table tr td span'
TagSemanticPath: 'is exactly' 'my-custom-element div div span:last'
TagSemanticPath: 'is exactly' 'my-custom-element div table tr:nthChild(2n +1)'
TagSemanticPath: 'is exactly' 'div div div div table tr td td td td'

Reverse traversals would also be very nice to have.