Automated web GUI testing has been widely adopted since manual testing is time-consuming and tedious. Waiting strategy plays a vital role in automated web GUI testing since it significantly impacts the testing performance. Though important, little focus has been set on the waiting strategies in web GUI testing. Existing waiting strategies either wait for a predetermined time, which is not reliable in a dynamic environment or only wait for a specific condition to be verified, which is often not robust enough to handle the complicated testing scenarios. In this work, we introduce a robust waiting strategy. Instead of waiting for a predetermined time or waiting for the availability of a particular element, our approach waits for a desired state to reach. This is achieved by capturing the Document Object Models (DOM) at the desired point, followed by an offline analysis to identify the differences between the DOMs associated with every two consecutive test actions. Such differences are used to determine the appropriate waiting time when automatically generating tests. Evaluation results with an industrial web application indicate that our approach produces more robust tests than the conventional waiting strategies used in web GUI testing. Furthermore, our generated tests are more representative of the recorded usage scenarios and are efficient with low overhead in test execution time.