Tiered browser support policy for Rust's web content

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+- Feature Name: tiered_browser_support
+- Start Date: 2017-04-25
+- RFC PR: https://github.com/rust-lang/rfcs/pull/1985
+- Rust Issue: https://github.com/rust-lang/rust/issues/43035
+
+# Summary
+[summary]: #summary
+
+Official web content produced by the Rust teams for consumption by Rust users
+should work in the majority of browsers that Rust users are visiting these
+sites in. The Rust compiler only supports [a finite number of targets][forge],
+with varying degrees of support, due to the limits on time, expertise, and
+testing resources. Similarly, we don't have enough time, expertise and testing
+resources to be sure that our web content works in every version of every
+browser. We should have a list of browsers and versions in various tiers of
+support.
+
+[forge]: https://forge.rust-lang.org/platform-support.html
+
+# Motivation
+[motivation]: #motivation
+
+[This pull request to remove JQuery from rustdoc's output][jquery-pr] had
+discussion about what we could and could not do because of browser support.
+This is a discussion we haven't yet had as a community.
+
+[jquery-pr]: https://github.com/rust-lang/rust/pull/41307
+
+Crates.io doesn't display correctly in browsers without support for flexbox,
+such as [Windows Phone 8.1][win-phone], a device that is no longer supported. I
+made the decision that it wasn't worth it for the community to spend time
+fixing this issue, did I make the correct tradeoff for the community?
+
+[win-phone]: https://github.com/rust-lang/crates.io/issues/56
+
+Supporting all versions of all browsers with the same behavior is impossible
+with the small number of people who work on Rust's web content. Crates.io is
+not currently doing any cross-browser testing; there are some JavaScript tests
+of the UI that run in [PhantomJS][], a headless WebKit. Since we're not
+testing, we don't actually know what our current web support even is, except
+for when we get bug reports from users.
+
+[PhantomJS]: http://phantomjs.org/
+
+In order to fully test on all browsers to be sure of our support, we would
+either need to have all the devices, operating systems, browsers, and versions
+available and people with the time and inclination to do manual testing on all
+of these, or we would need to be running automated tests on something like
+[BrowserStack][]. BrowserStack does appear to have a free plan for open source
+projects, but it's unclear how many parallel tests the open source plan would
+give us, and we'd at least be spending time waiting for test results on the
+various stacks. [BrowserStack also doesn't support every platform][bs-support],
+Linux on the desktop being a notable section of our userbase missing from their
+platforms.
+
+[BrowserStack]: https://www.browserstack.com/pricing
+[bs-support]: https://www.browserstack.com/support
+
+# Detailed design
+[design]: #detailed-design
+
+## Rust web content
+
+Officially produced web content includes:
+
+- rust-lang.org
+- blog.rust-lang.org
+- play.rust-lang.org
+- crates.io
+- Rustdoc output
+- thanks.rust-lang.org
+
+Explicitly not included:
+
+- Content for people working on Rust itself, such as:
+  - [The Rust Forge][]
+  - [rusty-dash][]
+
+[The Rust Forge]: https://forge.rust-lang.org/
+[rusty-dash]: https://rusty-dash.com/
+
+Things that are not really under our control but are used for official or
+almost-official Rust web content:
+
+- GitHub
+- docs.rs
+- Discourse (used for [urlo][] and [irlo][])
+- [mdBook][] output (used for the books and other documentation)
+
+[urlo]: https://users.rust-lang.org/
+[irlo]: https://internals.rust-lang.org/
+[mdBook]: https://github.com/azerupi/mdBook/
+
+## Proposed browser support tiers
+
+Based on [actual usage metrics][] and with a goal of spending our time in an
+effective way, the browser support tiers would be defined as:
+
+[actual usage metrics]: #google-analytics-browser-usage-stats
+
+Browsers are listed in [browserslist][] format.
+
+[browserslist]: https://github.com/ai/browserslist
+
+### Supported browsers
+
+Goal: Ensure functionality of our web content for 80% of users.
+
+Browsers:
+
+```
+last 2 Chrome versions
+last 1 Firefox version
+Firefox ESR
+last 1 Safari version
+last 1 iOS version
+last 1 Edge version
+last 1 UCAndroid version
+```
+
+[On browserl.ist](http://browserl.ist/?q=last+2+Chrome+versions%2C+last+1+Firefox+version%2C+Firefox+ESR%2C+last+1+Safari+version%2C+last+1+iOS+version%2C+last+1+Edge+version%2C+last+1+UCAndroid+version)
+
+Support:
+
+- We add automated testing of functionality in a variety of browsers through a
+  service such as [BrowserStack][] for each of these as much as possible (and
+  work on adding this type of automated testing to those web contents that
+  aren't currently tested, such as rustdoc output).
+- Bugs affecting the functionality of the sites in these browsers are
+  prioritized highly.
+
+### Unsupported browsers
+
+Goal: Avoid spending large amounts of time and code complexity debugging and
+hacking around quirks in older or more obscure browsers.
+
+Browsers:
+
+- Any not mentioned above
+
+Support:
+
+- No automated testing for these.
+- Bug reports for these browsers are closed as WONTFIX.
+- Pull requests to fix functionality for these browsers are accepted only if
+  they're deemed to not add an inordinate amount of complexity or maintenance
+  burden (subjective, reviewers' judgment).
+
+## Orthogonal but related non-proposals
+
+The following principles are assumptions I'm making that we currently follow
+and that we should continue to strive for, no matter what browser support
+policy we end up with:
+
+- Follow best practices for accessibilty, fix bug reports from blind users,
+  reach out to blind users in the community about how the accessibility of the
+  web content could be improved.
+  - This would include supporting lynx/links as these are sometimes used with
+    screen readers.
+- Follow best practices for colorblindness, such as have information conveyed
+  through color also conveyed through an icon or text.
+- Follow best practices for making content usable from mobile devices with a
+  variety of screen sizes.
+- Render content without requiring JavaScript (especially on
+  [crates.io][noscript]). Additional functionality beyond reading (ex: search,
+  follow/unfollow crate) may require JavaScript, but we will attempt to use
+  links and forms for progressive enhancement as much as possible.
+
+[noscript]: https://github.com/rust-lang/crates.io/issues/204
+
+Please comment if you think any of these should **not** be assumed, but rest
+assured it is not the intent of this RFC to get rid of these kinds of support.
+
+# Relevant data
+
+[CanIUse.com][] has some statistics on global usage of browsers and versions,
+but our audience (developers) isn't the same as the general public.
+
+[CanIUse.com]: http://caniuse.com/usage-table
+
+## Google analytics browser usage stats
+
+We have Google Analytics on crates.io and on rust-lang.org. The entire data set
+of the usage stats by browser, browser verison, and OS are available [in this
+Google sheet][all-data] for the visits to crates.io in the last month. I chose
+to use just crates.io because on initial analysis, the top 90% of visits to
+rust-lang.org were less varied than the top 90% of visits to crates.io.
+
+[all-data]: https://docs.google.com/spreadsheets/d/1qgszm-_-Tn8FLi2v3vicuvyct3Grzz74JWcroILRq8s/edit?usp=sharing
+
+This data does not include those users who block Google Analytics.
+
+This is the top 80% aggregated by browser and major browser version:
+
+| Browser         | Browser Version | Sessions | % of sessions | Cumulative % |
+|-----------------|-----------------|----------|---------------|--------------|
+| Chrome          | 57              | 18,040   | 34.85         | 34.85        |
+| Firefox         | 52              | 8,136    | 15.72         | 50.56        |
+| Chrome          | 56              | 7302     | 14.11         | 64.67        |
+| Safari          | 10.1 (macos)    | 1,592    | 3.08          | 67.74        |
+| Safari          | 10 (ios)        | 1,437    | 2.78          | 70.52        |
+| Safari          | 10.0.3 (macos)  | 851      | 1.64          | 72.16        |
+| Firefox         | 53              | 767      | 1.48          | 73.65        |
+| Chrome          | 55              | 717      | 1.39          | 75.03        |
+| Firefox         | 45              | 693      | 1.34          | 76.37        |
+| UC Browser      | 11              | 530      | 1.02          | 77.40        |
+| Chrome          | 58              | 520      | 1.00          | 78.40        |
+| Safari (in-app) | (not set) (ios) | 500      | 0.97          | 79.37        |
+| Firefox         | 54              | 472      | 0.91          | 80.28        |
+
+Interesting to note: Firefox 45 is the latest
+[ESR](https://www.mozilla.org/en-US/firefox/organizations/all/) (Firefox 52
+will also be an ESR but it was just released). Firefox 52 was the current major
+version for most of this past month; I'm guessing the early adopters of 53 and
+54 are likely Mozilla employees.
+
+## What do other sites in our niche support?
+
+- [GitHub][] - Current versions of Chrome, Firefox, Safari, Edge and IE 11.
+  Best effort for Firefox ESR.
+- [Discourse][] - Chrome 32+, Firefox 27+, Safari 6.1+, IE 11+, iPad 3+, iOS
+  8+, Android 4.3+ (doesn't specify which browser on the devices, doesn't look
+  like they've updated these numbers in a while)
+
+[GitHub]: https://help.github.com/articles/supported-browsers/
+[Discourse]: https://github.com/discourse/discourse#requirements
+
+# How We Teach This
+[how-we-teach-this]: #how-we-teach-this
+
+We should call this "Rust Browser Support", and we should have the tiers listed
+on the Rust Forge in a similar way to the tiers of Rust platforms supported.
+
+We should link to the tiered browser support page from places where Rust web
+content is developed and on the [Rust FAQ][].
+
+[Rust FAQ]: https://www.rust-lang.org/en-US/faq.html
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+We exclude some people who are unwilling or unable to use a modern browser.
+
+# Alternatives
+[alternatives]: #alternatives
+
+We could adopt the tiers proposed above but with different browser versions.
+
+We could adopt the browsers proposed above but with different levels of support.
+
+Other alternatives:
+
+## Not have official browser support tiers (status quo)
+
+By not creating offical levels of browser support, we will continue to have the
+situation we have today: discussions and decisions are happening that affect
+the level of support that Rust web content has in various browsers, but we
+don't have any agreed-upon guidelines to guide these discussions and decisions.
+
+We continue to not test in multiple browsers, instead relying on bug reports
+from users. The people doing the work continue to decide on an ad-hoc basis
+whether a fix is worth making or not.
+
+## Support all browsers in all configurations
+
+We could choose to attempt to support any version of any browser on any device,
+testing with as much as we can. We would still have to rely on bug reports and
+help from the community to test with some configurations, but we wouldn't close
+any bug report or pull request due to the browser or version required to
+reproduce it.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+- Am I missing any official web content that this policy should apply to?
+- Is it possible to add browser tests to rustdoc or would that just make the
+  current situation of long, flaky rustc builds worse?