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Performance Timeline API defines extensions to the Performance interface to support client-side latency measurements within applications. Extensions provide interfaces to retrieve performance entry metrics based on specific filter criteria. The standard also includes interfaces that allow an application to define performance observer callbacks that are notified when specific performance events are added to the browser's performance timeline. The PerformanceEntry interface encapsulates a single performance entry — that is, a single data point or metric in the performance timeline. This document provides an overview of the standard's interfaces.