RFC(IE11): Ending support for IE11 in Fluent v9 (#18978)

* RFC(IE11): Ending support for IE11 in Fluent v9

* update proxy usage

* Update rfcs/convergence/ending-ie11-support.md

Co-authored-by: Martin Hochel <[email protected]>

* Update rfcs/convergence/ending-ie11-support.md

Co-authored-by: Martin Hochel <[email protected]>

* update pros/cons

* update RFC

Co-authored-by: Martin Hochel <[email protected]>

rfcs/convergence/ending-ie11-support.md

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+# RFC: Ending support for IE11 with Fluent v9
+
+@ling1726
+
+## Summary
+
+Fluent v9 will officially no longer support IE11.
+
+[Microsoft has officially announced a deadline to end IE11 support for M365 Apps](https://techcommunity.microsoft.com/t5/microsoft-365-blog/microsoft-365-apps-say-farewell-to-internet-explorer-11-and/ba-p/1591666).
+This comes at a time where the market share of IE11 has [dropped to below 1%](https://caniuse.com/usage-table).
+
+One of the major goals of the convergence project in Fluent v9, is to address past drawbacks and issues with both `@fluentui/react` and `@fluentui/react-northstar` and
+improve the experience for web developers at Microsoft. This has given an opportunity for the team to take a step back, look at the
+fundamentals and rethink our support for IE11.
+
+In this RFC we will explain our motivations for this decision and finally formalize this decision in writing.
+
+## Detailed Design or Proposal
+
+### Keep calm and carry on
+
+Our partners and products in M365 will either adhere to the August 2021 deadline or receive a 6 month extension for IE11 support.
+Since Fluent v9 is still in alpha release, and is planned to be at beta at August 2021, we will be ready for early adopters.
+Until that deadline, users that require full support for IE11 can do so with the existing versions of `@fluentui/react` and
+`@fluentui/react-northstar` that do provide support.
+
+## Motivation
+
+### Reduced transpilation - (perf + bundlesize improvements)
+
+The ability to use ES2015/ES6 features without the transpilation required for IE11 will result in perf and bundle size improvements,
+and tslib will no longer be needed as a dependency to emulate the missing features of modern javascript.
+
+It's possible that newer language features in the future might re-introduce tslib or extra transpilation. However since our support moves
+mainly to evergreen browsers, the risk of introducing unsupported language features will decrease.
+
+### Improved Theming
+
+Theming a web application is always a challenge and can easily lead to performance issues. Performance has been a challenge for the current
+versions of fluent that evaluated theme values with javascript. Fluent v9 uses [CSS variables](https://developer.mozilla.org/en-US/docs/Web/CSS/--*) for its theming and styling solution. This means
+that it does not need to have a different set of classes for each theme and css rules never change regardless of theme which is a
+big boost to performance.
+
+Our future theming system also uses the [Proxy](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy)
+in a key step to create the CSS variables that map to theme values.
+
+Without these platform features missing in IE11, we would need to dedicate more time to another solution which would be also less performant for both IE11 and evergreen browser users.
+to dedicate more time to another solution for this.
+
+### Maintenance Burden
+
+The lack of support that IE11 offers for modern (ES2015) javascript has not stopped developers from using it. With EOL of IE11
+we no longer have to consider the language features and built-in APIs used across the entire codebase, and figure out the the real support our
+transpiled code has for the browser. Since dropping support of IE11 is a breaking change, once we support it in a major version
+of Fluent, we can't drop support until the next major release.
+
+Our tooling will improve and its complexity will decrease. With babel and `preset-env` will be able to use the same tooling
+in our tests, storybook and the code we ship. Our build times and and test times will also decrease which means faster CI
+times overall.
+
+### Motivating our partners
+
+There are clear benefits we have planned with Fluent v9, from easier composition to performant styling. We want our partners
+to leverage the full set of advantages that Fluent v9 has to offer. The web ecosystem needs to move forward and supporting
+IE11 will inevitably make it slower. By taking this stance in Fluent v9 we give a clear motivation to our partners to
+modernize their own products.
+
+## Pros and Cons
+
+### Pros
+
+Already mentioned in previous chapters
+
+### Cons
+
+We understand that this might cause initial adoption headaches from our partners. However, they should also generally be targeting
+the same EOL date for IE11.
+
+There will be a cost to overhauling the codebase and tooling for vNext to fully leverage all the benefits that IE11 EOL will bring us.
+This cost is acceptable to the team, in achieving our efforts to make Fluent v9 a great modern design system.