OOXML was provided with Open Office 2007 and ODF in Service Pack 2. However, it seems likely that many documents will still be created in the older proprietary .DOC (Word 97-2003) format to maintain compatibility with older versions of Word and other pre XML formats.
Neither OOXML nor ODF can be read directly by a web browser. My preference would be for a HTML based format. Microsoft tried this with a previous version of Word, with a HTML based format which could be edited, however it did not become popular. However, newer versions of HTML and CSS provide more features and so fewer workarounds to implement word processor features. In addition both OOXML and ODF are conceptually similar to HTML based ebook formats, such as Epub and the Amazon Kindle format. These all consist of an XML formatted document, plus formatting and images, zipped into one file.