Title: Adversarial Environment Generation for Learning  to Navigate the Web. Abstract: Learning to autonomously navigate the web is a difficult sequential decision making task. The state and action spaces are large and combinatorial in nature, and successful navigation may require traversing several partially-observed pages. One of the bottlenecks of training web navigation agents is providing a learnable curriculum of training environments that can cover the large variety of real-world websites. Therefore, we propose using Adversarial Environment Generation (AEG) to generate challenging web environments in which to train reinforcement learning (RL) agents. We introduce a new benchmarking environment, gMiniWoB, which enables an RL adversary to use compositional primitives to learn to generate complex websites. To train the adversary, we present a new decoder-like architecture that can directly control the difficulty of the environment, and a new training technique Flexible b-PAIRED. Flexible b-PAIRED jointly trains the adversary and a population of navigator agents and incentivizes the adversary to generate ”just-the-right-challenge” environments by simultaneously learning two policies encoded in the adversary’s architecture. First, for its environment complexity choice (difficulty budget), the adversary is rewarded with the performance of the best-performing agent in the population. Second, for selecting the design elements the adversary learns to maximize the regret using the difference in capabilities of navigator agents in population (flexible regret). The results show that the navigator agent trained with Flexible b-PAIRED generalizes to new environments, significantly outperforms competitive automatic curriculum generation baselines—including a state-of-the-art RL web navigation approach and prior methods for minimax regret AEG—on a set of challenging unseen test environments that are order of magnitude more complex than the previous benchmarks. The navigator agent achieves more than 75% success rate on all tasks, yielding 4x higher success rate that the strongest baseline.