See #2723 for 2.2 version and more info.

Questionable and complicated reverts are the same, but just in case. I don't have access to IE6-7 at the moment, so i'm having trouble reverting 90d7cc1 commit. Help would be appreciated.

Used different revert tactic this time, so in the tests there is places like -

test( "...."
// and
QUnit.test( "..."

etc.

Mostly, i propose for you to check if API is the same as in 2.2 branch.

Reverted commits

Reason: Behaviour change
Name: Attributes: do not set properties to false when removing booleans
Commit: 5c086c3
Issue: #1759

Reason: Behaviour change
Name: Core: make isNumeric limited to strings and numbers
Commit: ada073e
Issue: #2662

Reason: Follow-up commit for 9df8bd2
Name: CSS: Make show/hide/toggle methods a module
Commit: 3842246
Issue: #2193
PR: #2648

Reason: API change (signature)
Name: Effects: Remove additional parameters of easings
Commit: 41a04e5
Issue: #2367
PR: #2376

Reason: API change (new method)
Name: Deferred: add .catch handler
Commit: ef77f83
Issue: #2102

Reason: Follow-up commit for ef30bdf
Name: Ajax: Remove remnants of the load event alias handling
Commit: 38bd5fe

Reason: API change (removal of public methods)
Name: Event: remove deprecated event aliases
Commit: ef30bdf
Issue: #2286
PR: #2287

Reason: Behaviour change
Name: CSS: Ignore the CSS cascade in show()/hide()/etc.
Commit: 9df8bd2
Issue: #1767
Issue: #2071
PR: #2180

Reason: Behaviour change
Name: Offset: allow offset setter to throw for disconnected elements
Commit: dc49f62
Issue: #2114

Reason: Behaviour change
Name: Offset: remove ownerDocument check in offset getter
Commit: a2386a8
Issue: #2115

Reason: Behaviour change
Name: Offset: Fix .offset() to correctly work with ShadowDOM
Commit: d4dd548
Issue: #1784
PR: #2043

Reason: Behaviour change
Name: Data: camelCasing should not ignore case
Commit: cf16f86
Issue: #2070

Reason: Behaviour change
Name: Data: always camelCase keys in .data()
Commit: 0204c30
Issue: #2257

Reason: Behaviour change
Name: Data: do not include digits when camelCasing
Commit: a254f22
Issue: #1751

Reason: Behaviour change
Name: Data: do not include digits when camelCasing
Commit: a254f22
Issue: #1751

Reason: API change (removal of public methods)
Name: Ajax: remove deprecated extensions from ajax promise
Commit: 9368a8c
Issue: #2084
PR: #2092

Reason: IE6-7 compliance
Name: Manipulation: don't auto-insert tbody
Commit: 939e755
Issue: #1835
PR: #2021

Reason: Behaviour change
Name: Callbacks: Don't abort execution on .lock()
Commit: fc7477f
Issue: #1990
PR: #1991

Reason: IE6-7 compliance
Name: CSS: simplify hack of css getter for the computed values
Commit: dac716c
Issue: #1906

Reason: Follow-up commit for 90d7cc1
Name: CSS: simplify "defaultDisplay" module
Commit: d9d8906

Reason: Follow-up commit for 90d7cc1
Name: Traversing: simplify jQuery#contents method
Commit: 7230df1

Reason: Behaviour change
Name: Manipulation: make wrapAll funarg execute only once
Commit: 3dcee02
Issue: #1843

Reason: Behaviour change
Name: Core: Throw an error on $("#") rather than returning 0-length collection
Commit: 93f95c9
Issue: #1682

Reason: API change (removal of public properties)
Name: Core: Remove deprecated context and selector properties
Commit: e2ec5da
Issue: #1908
PR: #2000

Reason: Follow-up commit for 06a4540
Name: Effects: Improve raf logic
Commit: 9dc29a2

Reason: Behaviour change
Name: Effects: Reintroduce use of requestAnimationFrame
Commit: 06a4540
Issue: trac-15147

Reason: Behaviour change
Name: Ajax: Always use script injection in globalEval
Commit: 37f0f7f
Issue: trac-14757

Reason: Old Opera compliance
Name: Ajax: simplify one ajax call and add explanatory comment
Commit: 4b3e630

Reason: IE6-7 compliance
Name: Manipulation: simplification of manipulation wrappers
Commit: 3e87120
PR: #2032

Reason: API change (different return value)
Name: Core: Return empty array instead of null for parseHTML("")
Commit: 61bb612
Issue: #1997
PR: #1998

Reason: API change (signature change {feature})
Name: Wrap: Support .unwrap( selector) for selective unwrapping
Commit: e38138a
Issue: #1744
PR: #2003

Also removed effects tests for jQuery.Animate and jQuery.Tween by cherry-picking
a5864ae

/cc @jquery/core