In a blog post, Microsoft stated:
These APIs allow websites to convert text to audible speech with customizable voice and language settings. With them, website developers can add and control text-to-speech features specific to their page content and design. Speech Synthesis is useful whenever narration might be applied. Our implementation also supports Speech Synthesis Markup Language (SSML) Version 1.0 to provide further control over the speech output.
Windows 10 Insiders can check out how these Speech Synthesis APIs work in Edge by going to the TestDrive demo site and type in some words that can be generated into sound with this new feature. The APIs will be included for all Windows 10 users as part of the big Anniversary Update later this summer.