Add new page describing our browser support strategy

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+---
+title: Browser support
+weight: 100
+---
+
+# Browser support
+
+To make sure the public can successfully access and use government services, regardless of the browser they're using, the Design System team has divided browsers into 4 grades. Each grade shows the level of support we will provide.
+
+You can see more information about [how we provide support for different browsers](https://github.com/alphagov/govuk-frontend/blob/main/docs/contributing/browser-support.md) in our GitHub documentation.
+
+## Grade A
+
+Grade A browsers include the most recent stable versions of Chrome, Firefox, Edge, Samsung Internet and Safari.
+
+These browsers should be able to parse GOV.UK Frontend's JavaScript without error. We aim to provide the same overall experience in Grade A and B browsers.
+
+When supporting these browsers we will:
+
+-   use Grade A browsers for any manual testing carried out during the development process
+-   use our automated test suites as standard
+-   treat bugs affecting Grade A browsers as high priority
+
+## Grade B
+
+Grade B browsers include all stable versions of Chrome, Firefox and Edge released in the last 6 months and the last 4 releases of Safari which are not supported in Grade A.
+
+These browsers should be able to parse GOV.UK Frontend's JavaScript without error. We aim to provide the same overall experience in Grade A and B browsers.
+
+When supporting these browsers we will:
+
+-   use our automated test suites as standard
+-   treat bugs affecting Grade B browsers as low priority unless they prevent a user from being able to complete their task
+
+## Grade C
+
+Grade C covers browsers not in Grade A or B which support `<script type="module">`. These are:
+
+-   Chrome 61+
+-   Edge 16-18
+-   Edge 79+
+-   Safari 11 (mac)
+-   Firefox 60+
+-   Opera 48+
+-   Safari 10.3+ (iOS)
+-   Samsung Internet 8.2+
+
+Safari 10.1 also supports `<script type="module">` but will 'exit early' as it does not support `HTMLScriptElement.prototype.nomodule` which is how we test support for `<script type="module">` from within our JavaScript.
+
+These browsers should be able to parse GOV.UK Frontend's JavaScript without error. 
+
+However, we might disable or reduce features on an individual basis in browsers where the underlying features are not available. Exceptions will be if users need the feature to complete their task.
+
+For grade C browsers we:
+
+-   might remove support for individual features in these browsers at any time without considering them a breaking change
+-   will not regularly test in these browsers
+-   will not fix bugs affecting these browsers unless they prevent a user from being able to complete their task, or we can fix the bug by disabling a feature
+
+## Grade X
+
+Grade X includes all browsers that do not support `<script type="module">`, including all versions of Internet Explorer.
+
+These browsers should not download or attempt to parse GOV.UK Frontend's JavaScript.
+
+We will not regularly test in these browsers. We will not fix bugs affecting these browsers.