Unlike ggplot2 where interactivity is bolted on via ggplotly(), glyph treats interaction as part of the visualization grammar. Interactions are declared in the spec and compiled to the appropriate backend (D3 events for HTML, Shiny bindings for server-side).
Details
Design philosophy: interactions are selections that filter or highlight marks. This follows Vega-Lite's insight that most interactions are really about defining subsets of the data. A tooltip is "select the nearest point, show its values." Brushing is "select points in a rectangle, highlight them."