A common Event problem may be the cause of widespread test failures across components for IE 9.
These are a sample of test failures:
✖ yuitests1286931021379 on Internet Explorer (9.0) / Windows
5 passed, 12 failed
in DOM Event Tests
test_on_with_string click handler via Y.on didn't work when passed a string.
Expected: true (boolean)
Actual: false (boolean)
test_on_with_node click handler via Y.on didn't work when passed a node.
Expected: true (boolean)
Actual: false (boolean)
test_node_on click handler didn't work
Expected: true (boolean)
Actual: false (boolean)
test_nodelist_on click handler didn't work
Expected: 1 (number)
Actual: 0 (number)
test_y_on_with_nodelist click handler didn't work
Expected: 1 (number)
Actual: 0 (number)
test_y_detach_with_nodelist click handler didn't work
Expected: 1 (number)
Actual: 0 (number)
test_y_detach_with_array click handler didn't work
Expected: 1 (number)
Actual: 0 (number)
test_y_on_with_selector_multiple_match click handler didn't work
Expected: 1 (number)
Actual: 0 (number)
test_y_on_with_htmlelementcollection click handler didn't work
Expected: 1 (number)
Actual: 0 (number)
test_detach_by_sig detach by signature was a failure
Expected: 1 (number)
Actual: 0 (number)
test_purge Values should be equal.
Expected: 1 (number)
Actual: 0 (number)
test_categories Values should be equal.
Expected: 2 (number)
Actual: 0 (number)
✖ Delegate on Internet Explorer (9.0) / Windows
4 passed, 15 failed
in Event Delegate Tests
test_simple_delegate_for_anchors simple delegation fails, mod1 should pickup the event and test target [firstlink]
Expected: true (boolean)
Actual: false (boolean)
test_multiple_selectors event delegate works but the matched element is an incorrect node, should be the matching node
Expected: undefined (undefined)
Actual: DIV#mod-header ms__id5287 (object)
test_filter_function Values should be the same.
Expected: 1 (number)
Actual: 0 (number)
test_document_as_container simple delegation fails, mod1 should pickup the event and test target [firstlink]
Expected: true (boolean)
Actual: false (boolean)
test_checking_delegation_target delegation fails for an image within an anchor, mod1 should pickup the event and test target [secondlink]
Expected: true (boolean)
Actual: false (boolean)
test_multiple_selectors_one_match multiple selectors fails, delegate should be able to match different selectors
Expected: true (boolean)
Actual: false (boolean)
test_multiple_delegate_matches first match fail, delegate should be able to match [a]
Expected: true (boolean)
Actual: false (boolean)
test_bubble_up_after_delegate first match fail, delegate should be able to match [a]
Expected: true (boolean)
Actual: false (boolean)
test_bubble_up_after_delegate_halt first match fail, delegate should be able to match [a]
Expected: true (boolean)
Actual: false (boolean)
test_direct_descendant_combinator The currentTarget is an incorrect node, should be the matching node.
Expected: list-2-li-3 (string)
Actual: null (object)
test_multiple_matches_in_subtree Delegate handler should have fired three times, once for each div in #div-1
Expected: 3 (number)
Actual: 0 (number)
test_removing_parent_in_subscriber Values should be the same.
Expected: 2 (number)
Actual: 0 (number)
test_successful_purge The second delegated listener should be called.
Expected: true (boolean)
Actual: false (boolean)
test_successful_detach The second delegated listener should be called.
Expected: true (boolean)
Actual: false (boolean)
in Bugs
stopPropagation should stop multiple matches Value should be true.
Expected: true (boolean)
Actual: false (boolean)
✖ Y.ValueChange on Internet Explorer (9.0) / Windows
1 passed, 7 failed
in Everything
valueChange event should start polling on mousedown and fire an event when the value changes Value should be true.
Expected: true (boolean)
Actual: undefined (undefined)
valueChange should support textareas as well Value should be true.
Expected: true (boolean)
Actual: undefined (undefined)
valueChange should start polling on keydown Value should be true.
Expected: true (boolean)
Actual: undefined (undefined)
valueChange should stop polling on blur Value should be true.
Expected: true (boolean)
Actual: undefined (undefined)
valueChange should start polling on focus Value should be true.
Expected: true (boolean)
Actual: undefined (undefined)
valueChange should start polling on keyup for IME keystrokes Value should be true.
Expected: true (boolean)
Actual: false (boolean)
valueChange should stop polling after timeout Value should be true.
Expected: true (boolean)
Actual: undefined (undefined)
Reid Burke 
Adam Moore 
DOM is all good across browsers.
Delegate is greatly improved but there's a new cross-browser issue which includes IE 9:
✖ Delegate on Safari (5.0.2) / Mac OS
17 passed, 2 failed
in Event Delegate Tests
test_multiple_matches_in_subtree: First delegate callback should be from the highest div in the subtree
Expected: div-1-2 (string)
Actual: div-1-2-1-1 (string)
test_removing_parent_in_subscriber: Values should be the same.
Expected: 2 (number)
Actual: 1 (number)
✖ Delegate on Firefox (4.0b6) / Windows
17 passed, 2 failed
in Event Delegate Tests
test_multiple_matches_in_subtree: First delegate callback should be from the highest div in the subtree
Expected: div-1-2 (string)
Actual: div-1-2-1-1 (string)
test_removing_parent_in_subscriber: Values should be the same.
Expected: 2 (number)
Actual: 1 (number)
✖ http://172.21.155.36:8000/project/4844612/Users/reid/Development/yui/yui3/src/event/tests/delegate.html on Internet Explorer (9.0) / Windows
0 passed, 1 failed
in window.onerror handler (yeti virtual test)
window.onerror should not fire: 'null' is null or not an object
✖ Delegate on Internet Explorer (9.0) / Windows
17 passed, 2 failed
in Event Delegate Tests
test_multiple_matches_in_subtree: First delegate callback should be from the highest div in the subtree
Expected: div-1-2 (string)
Actual: div-1-2-1-1 (string)
test_removing_parent_in_subscriber: Values should be the same.
Expected: 2 (number)
Actual: 1 (number)