Progressive Web Apps (PWA) are a new class of Web applications, enabled for the most part by the Service Workers APIs. Service Workers allow apps to work offline by intercepting network requests to deliver programmatic or cached responses, Service Workers can receive push notifications and synchronize data in the background even when the app is not running, and—together with Web App Manifests—allow users to install PWAs to their devices’ home screens. Service Workers being a Web standard, support has landed in several stand-alone Android Web browsers—among them (but not limited to) Chrome and its open-source foundation Chromium, Firefox, Edge, Opera, UC Browser, Samsung Internet, and—eagerly awaited—iOS Safari. In this paper, we examine the PWA feature support situation in Web Views, that is, in-app Web experiences that are explicitly not stand-alone browsers. Such in-app browsers can commonly be encountered in chat applications like WeChat or WhatsApp, online social networks like Facebook or Twitter, but also email clients like Gmail, or simply anywhere where Web content is displayed inside native apps. We have developed an open-source application called PWA Feature Detector that allows for easily testing in-app browsers (and naturally stand-alone browsers), and have evaluated the level of support for PWA features on different devices and Web Views. On the one hand, our results show that there are big differences between the various Web View technologies and the browser engines they are based upon, but on the other hand, that for Android the results are independent from the devices’ operating systems, which is good news given the problematic update policy of many device manufacturers. These findings help developers make educated choices when it comes to determining whether a PWA is the right approach given their target users’ means of Web access.