A boundary condition is a limit or edge case that software must handle correctly, such as a value at the end of a range or an unusually sized input. In browser layout code, boundary checks help ensure that page elements are measured and arranged safely and accurately.
When code handles a boundary incorrectly, it may read or write outside an intended area of memory, potentially causing a crash, exposing data, or enabling code execution. Attackers may craft web content that drives vulnerable code into such an edge case. Developers reduce this risk with careful input validation, safe memory handling, and tests for boundary values; users and administrators should install security updates that fix confirmed flaws.