Long gone (hopefully) are the days of replicating the "professionals only" desktop GIS interface in a browser. However, with modern browsers, HTML5 APIs, and increased efficiency of javascript engines it is possible to performantly replicate GIS functionality in a purely client-side browser application. Moderately complex geoprocessing, persistent client-side storage and simple to complex data visualization are all possible now. We walk through the underlying technology and demonstrate the practical use of it in an open-source sample application. Technologies covered include IndexedDB, WebStorage, Workers, Strongly Typed Arrays and Canvas. Some attention will also be paid to performance limitations, browser support and polyfills for older browsers.