Currently, the official Adaptive Cards Designer uses enum values for validating the JSON, meaning that spacing: "left" will cause a linter warning in the AC designer but spacing: "Left" will pass.

In the old Adaptive Cards designer, all lowercase values were supported as opposed to capitalized values, which is part of why the cards package lints for lowercase.

Ultimately, either works - capitalized or lowercase, either/both will successfully be rendered in Teams over the wire, but to avoid confusion for the conflicting linter messages, this SDK should support both. (This is why the work item is lower priority)

Adaptive Cards Designer, lowercase:

AC Designer, capitalized:

Linter when using ICard from cards package: