EPUB is an interchange format for digital documents that can represent and package together web content like HTML, CSS, SVG, and other resources.
Version 3.3 is backwards compatible with the previous version 3.2, so documents created in 3.2 are also valid in 3.3 and no changes need to be made for document workflows.
Becoming a W3C Recommendation means that EPUB 3.3 is now a web standard. The process for standardization includes a thorough review by W3C members, which often leads to important technical changes. Recommandations also include detailed guidelines for implementation.
During the standardization process, W3C also creates a test suite, which they have done for EPUB 3.3.
The W3C has also significantly updated the EPUB documentation. The content specification has been separated from the reading system specifications, because usually there are two different groups of people interested in each. They have also worked to make the documents more readable.
Work was also done to improve accessibility. The EPUB Accessibility specification was updated and made part of the EPUB Standard. According to the W3C, the specification meets the requirements of the European Accessibility Act.
Going forward, the W3C will maintain the EPUB specification and a working group has been formed to consider development of new features, though they say no major technical changes are planned at the moment.