Kathy Brown summed up the most important changes:

Domino Designer 8.5.3 (and the Notes client, and the admin client) launched October 4, 2011, and some nice new features were added.

A small new feature, but one that I really like is the addition of new perspectives in Domino Designer.  You can go to Window, Open Perspective -- and choose an XPages perspective or a Forms/Views perspective. The XPages perspective removes items like framesets and pages that are not necessary for developing XPages. The Forms/Views perspective accordingly shows those elements necessary for client development and removes those not required, like custom controls and XPages.

Source control enablement was added to 8.5.3, which allows developers to store design elements in a source control repository. The enablement allows for integration in an effort to keep the source control "system agnostic".

There is an updated JavaScript editor with improved code validati

on, the ability to set formatting preferences, code templates, and other improvements.

New palette profiles allow you to customize what appears in the control palette. Maybe you never use radio buttons, so now you can remove them from your profile palette. Go to File --> Preferences --> Domino Designer --> Palette to customize and/or reset.

Small, but oh so useful usability improvements include:

    *  A sign button in all design list editors

    *  Ability to control opening of info boxes: With a preference you
       can now stop the info box from auto-opening at random times.
       It may make sense to someone, but more often than not I find
       myself annoyed when it pops up some times, and not others.

    *  Ability to rename a design element AND its alias -- renaming
       the alias dropped off from the rename dialog and finally, it is
       back

    *  Ability to "Expand All" and "Collapse All" in the Outline view

Dojo has been upgraded to 1.5.1 with release 8.5.3.  There is also an available plugin containing Dojo 1.6.1 for consumption by XPages.

Full text search results sorting is now supported in XPages.

HTML5 attributes have been added to XPage controls.

JavaScript/CSS aggregation was added. Multiple Dojo modules or multiple CSSs can be aggregated into a single file, improving performance.

OneUI 2.1 was included with 8.5.3, which includes several new colors to select from.

Symphony 3 was shipped with the Notes client in 8.5.3. This does not directly affect developers, but is something to be aware of if your applications export data to Symphony.

A new Java design element has been added to ease the use of Java in Domino applications. It allows the Java class to be scoped directly to the NSF, eliminating the need to manually put the files into the NSF.

A new @Function was added -- @GetMachineInfo. This function allows policies to be pushed based on machine, such as certain policies for laptop versus desktop.

The complete list and further details on what is new in 8.5.3 can be found in the release notes.