Viewports and Sub-Pages for Web-Based User Interfaces

An embodiment may involve web page metadata that defines a web page, first sub-page metadata that defines a first sub-page, and second sub-page metadata that defines a second sub-page, wherein the web page metadata includes specification of a viewport, and wherein the viewport is associated with an identifier. One or more processors may be configured to: receive a request for the web page; resolve the web page metadata into web content, wherein resolving the web page metadata includes: (i) determining, based on the identifier, a route associated with the viewport, (ii) determining, based on the route, a set of conditions associated with the viewport, (iii) determining that a particular condition is satisfied, wherein the particular condition is associated with the first sub-page and (iv) placing, based on the particular condition being satisfied, the first sub-page metadata in the viewport; and transmit the web content.