The Expression Web team has just announced the availability of Expression Web 4 SP1. This service pack contains significant fixes and improvements in publishing, in the SuperPreview feature, in file management, extensibility and also adds support for IE9, HTML 5 and CSS 3.
Key Improvements

• Support for all HTML 5 tags (for pages authored using the HTML 5 doctype) via IntelliSense within the code editor, and as block or inline entities in the Design view.
• Support for the draft CSS 3 specification.
• Improved SuperPreview now supports cookie-based logon authentication, now you can preview and access to Pages Protected by Authentication Forms.
• PHP IntelliSense support for the latest functions from PHP 5.3, as well as support for 130+ additional PHP modules.
• SuperPreview includes support for Chrome, IE8, IE9, Safari 4 and 5 on Windows and Mac. Because it is using the remote service, so you don’t need to have these browsers installed locally.
• JavaScript (.js) files now display IntelliSense for methods associated with the document object.
• The new option allows Expression Web to recognize PHP files even if they don’t have a PHP extension.
• New IntelliSense is included for extended HTML characters
• Additionally Open as PHP, Copy Full Path command and Open Folder in Windows Explorer options added.

Microsoft Expression Web 4 Service Pack 1 Download | Release Notes