With 17.08 we reach a 90+% level of feature readiness with our new HTML5 Fiori UI. Please keep in mind, that certain capabilities, e.g. Mashups will take some more time to be fully available.
Never the less, this is the right time to start working on your transition plan from Silverlight to HTML5.
Please find enclosed a short video about the evolution of the UI, where we are right now and some forward looking statements.