Forgive the second top-level comment, but I have some thoughts on Narrator and Edge. Disclosure: I worked on the Windows accessibility team at Microsoft during the transition from EdgeHTML to Chromium, and as a third-party screen reader developer before that. But I won't divulge anything confidential here.
It probably comes as no surprise that EdgeHTML and Chromium have completely different accessibility implementations. Narrator always had the best support for EdgeHTML. I was a third-party screen reader developer when EdgeHTML first came out, and for us third-party developers, EdgeHTML was a drastic change from IE. For over a decade, we had provided access to IE by injecting code into the IE process (yes, Windows lets you do that) and accessing the IE DOM in-process using COM. We did something similar for Firefox and Chromium, but using the IAccessible2 API (also COM-based). To improve security, old Edge disallowed this kind of injection; it could only be accessed through the UI Automation API. Narrator was built for this; the rest of us had to adapt after the fact. And since we could only access UIA through inter-process communication, not in-process like we did with the IE DOM and IAccessible2, there were performance problems, even with Narrator. (Luckily, I got to help solve those problems during my time on the Windows accessibility team.)
With Chromium (in both Google Chrome and the new Edge), screen readers can still inject code in-process and use the legacy IAccessible2 API. And NVDA, JAWS, and System Access (which I developed before joining Microsoft) do that. These third-party screen readers access Chrome and new Edge in the same way, at least inside the web content area, so if you're testing with one of these screen readers, it probably doesn't matter which browser you use. The situation with Narrator and Chromium-based browsers is more interesting. Narrator uses the UI Automation API to access all applications. Chromium has a native UIA implementation, largely contributed by the Edge team, but while that implementation is enabled by default in the new Edge, it isn't yet in Chrome. So Narrator accesses Edge using UIA. But for Chrome, and other Chromium-based apps (e.g. Electron apps), Narrator uses a bridge from IAccessible2 to UIA that's built into the UIA core module. So in corner cases, there may be differences in how Narrator behaves in Chrome and Edge.
So, should developers test with Narrator and/or Edge? Well, I may be too biased to answer that. But I think it's likely that Narrator usage is on the rise. While I was on the Narrator team at Microsoft, we heard from time to time about praise that Narrator was getting in the blind community. (Naturally I can't take full credit for that; it was a team effort.) Moreover, since Narrator is the option built into Windows, there will come a point (if it hasn't come already) when it's good enough for many users and they have no reason to seek a third-party alternative. Also, there are some PCs where Narrator is the only fully functional screen reader, specifically those running Windows 10 S (the variant that doesn't allow traditional side-loaded Win32 apps). I'd guess that an increasing number of students and users of corporate PCs are saddled with that variant of Windows. And while I can't say anything about future versions of Windows, one can make an educated guess based on the broader trajectory of the industry.
As for whether it's worth testing with Edge as opposed to Chrome, I don't know. Fortunately, browser usage data is readily available.
I did a bit more reading and noticed in the WebAIM stats that Narrator usage is indeed on the rise. Given that it is included in the OS, I would hope that it ends up as the defacto standard in much the same way as VoiceOver is for macOS.
Really interesting to hear some of the technical details from someone that worked on it - thank you!
It probably comes as no surprise that EdgeHTML and Chromium have completely different accessibility implementations. Narrator always had the best support for EdgeHTML. I was a third-party screen reader developer when EdgeHTML first came out, and for us third-party developers, EdgeHTML was a drastic change from IE. For over a decade, we had provided access to IE by injecting code into the IE process (yes, Windows lets you do that) and accessing the IE DOM in-process using COM. We did something similar for Firefox and Chromium, but using the IAccessible2 API (also COM-based). To improve security, old Edge disallowed this kind of injection; it could only be accessed through the UI Automation API. Narrator was built for this; the rest of us had to adapt after the fact. And since we could only access UIA through inter-process communication, not in-process like we did with the IE DOM and IAccessible2, there were performance problems, even with Narrator. (Luckily, I got to help solve those problems during my time on the Windows accessibility team.)
With Chromium (in both Google Chrome and the new Edge), screen readers can still inject code in-process and use the legacy IAccessible2 API. And NVDA, JAWS, and System Access (which I developed before joining Microsoft) do that. These third-party screen readers access Chrome and new Edge in the same way, at least inside the web content area, so if you're testing with one of these screen readers, it probably doesn't matter which browser you use. The situation with Narrator and Chromium-based browsers is more interesting. Narrator uses the UI Automation API to access all applications. Chromium has a native UIA implementation, largely contributed by the Edge team, but while that implementation is enabled by default in the new Edge, it isn't yet in Chrome. So Narrator accesses Edge using UIA. But for Chrome, and other Chromium-based apps (e.g. Electron apps), Narrator uses a bridge from IAccessible2 to UIA that's built into the UIA core module. So in corner cases, there may be differences in how Narrator behaves in Chrome and Edge.
So, should developers test with Narrator and/or Edge? Well, I may be too biased to answer that. But I think it's likely that Narrator usage is on the rise. While I was on the Narrator team at Microsoft, we heard from time to time about praise that Narrator was getting in the blind community. (Naturally I can't take full credit for that; it was a team effort.) Moreover, since Narrator is the option built into Windows, there will come a point (if it hasn't come already) when it's good enough for many users and they have no reason to seek a third-party alternative. Also, there are some PCs where Narrator is the only fully functional screen reader, specifically those running Windows 10 S (the variant that doesn't allow traditional side-loaded Win32 apps). I'd guess that an increasing number of students and users of corporate PCs are saddled with that variant of Windows. And while I can't say anything about future versions of Windows, one can make an educated guess based on the broader trajectory of the industry.
As for whether it's worth testing with Edge as opposed to Chrome, I don't know. Fortunately, browser usage data is readily available.