Support for Media Overlays in Readium Web will be implemented through the on-going work on Guided Navigation.

Media Overlays in EPUB 3.x are a subset of Guided Navigation where:

• resources in the readingOrder of the publication are exclusively using XHTML
• Guided Navigation may be available only on a portion of these resources
• the text is strictly referenced using HTML fragment IDs (chapter1.html#paragraph1)
• and audio resources are usually referenced with a start/end timestamp in their fragment (track1.mp3#t=0,15)

Support for associated styles is slightly undercooked in RWPM and will require some extra work.

Support will need to be spread out between both toolkits:

• the go-toolkit needs to be able to parse SMIL files and related metadata from the OPF of an EPUB and expose them in the Readium Web Publication Manifest created on the fly as Guided Navigation Documents
• the ts-toolkit needs to be able to detect the presence of these Guided Navigation Documents, fetch them and expose them using newly added models
• the ts-toolkit also needs a new audio navigator (which would be shared with audiobooks in the future), that will be used to read audio fragments and highlight the relevant text at the same time
• text highlighting needs to be implemented using the styles documented in the EPUB but should fallback to the Highlight API and default styles in case there isn't any authored styles