always indent html using tabs
dont toggle paused/overview modes needlessly when setting state
notes code format tweaks
limit how often the notes window updates presentation states
skip transitions in notes window for better main window performance
carry slide classes over to generated background elements
images with data-src attribute are now lazy-loaded #793
lazy loading support for video #793
tests for lazy loading
lazy-load support for audio #793
documentation for #793
null and type check what comes through postmessage