Page layout of content items from a variety of sources is performed. A content processing system queues content items, such as user-generated blogs, tweets, social networking status updates and other postings, received from a variety of sources. Each content item comprises one or more assets from one or more asset types. The asset types include text, images, and video. A page builder retrieves items from the queue and determines the item's size constraints. A template is selected from a template database to use as a layout for the items on a page; each template has a number of slots into which content can be placed. A layout module lays out the items into the slots of the selected template based on the respective size and aspect ratio constraints of the items, in order to build a page to serve to a client device.