Various features are provided for assisting users in efficiently locating and viewing network content of interest, including but not limited to particular portions of web pages. The features are particularly useful for users of mobile computing devices having a limited display size, but may be used with any type of client device. Some features identify popular content items by collectively analyzing the interactive behaviors of a population of users, such as zooming interactions. The results of such analysis can be used in various ways to improve users' browsing experiences. For example, popular content items can be highlighted on a web page, an option to zoom in automatically on popular content items can be provided, popular content items may be rendered at relatively high resolution, and caching of content items may be based partly on a measure of their popularity.