Software developers rely on the abundance of online resources to meet their informal learning needs. Designing systems to effectively support this relies on a sound understanding of the informal learning process. In this paper, we show how the development and implementation of an ontological model can be used to increase understanding of informal learning needs and enhance system design. We interviewed developers about their informal online learning behaviors, and created a novel ontological model, LILO (Learning Informally Online), to represent user interaction with online learning resources by incorporating the resulting concepts, typologies, and relationships. From this ontology, we derive five main design requirements for an informal learning system for developers: keyword support, search support, resource genre utilization, reliability cues, and objective support. We then demonstrate how the ontology can be applied to advance system design using as an example CAN (Composable Accessibility Infrastructure), a system capable of supporting web developers' informal online learning.