docs (#149): add attribute coercion section to migration guide

src/.vuepress/config.js

@@ -94,6 +94,7 @@ const sidebar = {
       children: [
         'migration/introduction',
         'migration/async-components',
+        'migration/attribute-coercion',
         'migration/custom-directives',
         'migration/custom-elements-interop',
         'migration/data-option',

src/guide/migration/attribute-coercion.md

@@ -0,0 +1,134 @@
+# Attribute Coercion Behavior
+
+::: info Info
+This is a low-level internal API change and does not affect most developers.
+:::
+
+## Overview
+
+Here is a high level summary of the changes:
+
+- Drop the internal concept of enumerated attributes and treat those attributes the same as normal non-boolean attributes
+- **BREAKING**: No longer removes attribute if value is boolean false. Instead, it's set as attr="false" instead. To remove the attribute, use null or undefined.
+
+For more information, read on!
+
+## 2.x Syntax
+
+In 2.x, we had the following strategies for coercing `v-bind` values:
+
+- For some attribute/element pairs, Vue is always using the corresponding IDL attribute (property): [like `value` of `<input>`, `<select>`, `<progress>`, etc](https://github.com/vuejs/vue/blob/bad3c326a3f8b8e0d3bcf07917dc0adf97c32351/src/platforms/web/util/attrs.js#L11-L18).
+
+- For "[boolean attributes](https://github.com/vuejs/vue/blob/bad3c326a3f8b8e0d3bcf07917dc0adf97c32351/src/platforms/web/util/attrs.js#L33-L40)" and [xlinks](https://github.com/vuejs/vue/blob/bad3c326a3f8b8e0d3bcf07917dc0adf97c32351/src/platforms/web/util/attrs.js#L44-L46), Vue removes them if they are "falsy" ([`undefined`, `null` or `false`](https://github.com/vuejs/vue/blob/bad3c326a3f8b8e0d3bcf07917dc0adf97c32351/src/platforms/web/util/attrs.js#L52-L54)) and adds them otherwise (see [here](https://github.com/vuejs/vue/blob/bad3c326a3f8b8e0d3bcf07917dc0adf97c32351/src/platforms/web/runtime/modules/attrs.js#L66-L77) and [here](https://github.com/vuejs/vue/blob/bad3c326a3f8b8e0d3bcf07917dc0adf97c32351/src/platforms/web/runtime/modules/attrs.js#L81-L85)).
+
+- For "[enumerated attributes](https://github.com/vuejs/vue/blob/bad3c326a3f8b8e0d3bcf07917dc0adf97c32351/src/platforms/web/util/attrs.js#L20)" (currently `contenteditable`, `draggable` and `spellcheck`), Vue tries to [coerce](https://github.com/vuejs/vue/blob/bad3c326a3f8b8e0d3bcf07917dc0adf97c32351/src/platforms/web/util/attrs.js#L24-L31) them to string (with special treatment for `contenteditable` for now, to fix [vuejs/vue#9397](https://github.com/vuejs/vue/issues/9397)).
+
+- For other attributes, we remove "falsy" values (`undefined`, `null`, or `false`) and set other values as-is (see [here](https://github.com/vuejs/vue/blob/bad3c326a3f8b8e0d3bcf07917dc0adf97c32351/src/platforms/web/runtime/modules/attrs.js#L92-L113)).
+
+The following table describes how Vue coerce "enumerated attributes" differently with normal non-boolean attributes:
+
+| Binding expression       | `foo` <sup>normal</sup> | `draggable` <sup>enumerated</sup> |
+| ------------------- | ----------------------- | --------------------------------- |
+| `:attr="null"`      | /                       | `draggable="false"`               |
+| `:attr="undefined"` | /                       | /                                 |
+| `:attr="true"`      | `foo="true"`            | `draggable="true"`                |
+| `:attr="false"`     | /                       | `draggable="false"`               |
+| `:attr="0"`         | `foo="0"`               | `draggable="true"`                |
+| `attr=""`           | `foo=""`                | `draggable="true"`                |
+| `attr="foo"`        | `foo="foo"`             | `draggable="true"`                |
+| `attr`              | `foo=""`                | `draggable="true"`                |
+
+We can see from the table above, current implementation coerces `true` to `'true'` but removes the attribute if it's `false`. This also led to inconsistency and required users to manually coerce boolean values to string in very common use cases like `aria-*` attributes like `aria-selected`, `aria-hidden`, etc.
+
+## 3.x Syntax
+
+We intend to drop this internal concept of "enumerated attributes" and treat them as normal non-boolean HTML attributes.
+
+- This solves the inconsistency between normal non-boolean attributes and “enumerated attributes”
+- It also makes it possible to use values other than `'true'` and `'false'`, or even keywords yet to come, for attributes like `contenteditable`
+
+For non-boolean attributes, Vue will stop removing them if they are `false` and coerce them to `'false'` instead.
+
+- This solves the inconsistency between `true` and `false` and makes outputing `aria-*` attributes easier
+
+The following table describes the new behavior:
+
+| Binding expression       | `foo` <sup>normal</sup>    | `draggable` <sup>enumerated</sup> |
+| ------------------- | -------------------------- | --------------------------------- |
+| `:attr="null"`      | /                          | / <sup>†</sup>                    |
+| `:attr="undefined"` | /                          | /                                 |
+| `:attr="true"`      | `foo="true"`               | `draggable="true"`                |
+| `:attr="false"`     | `foo="false"` <sup>†</sup> | `draggable="false"`               |
+| `:attr="0"`         | `foo="0"`                  | `draggable="0"` <sup>†</sup>      |
+| `attr=""`           | `foo=""`                   | `draggable=""` <sup>†</sup>       |
+| `attr="foo"`        | `foo="foo"`                | `draggable="foo"` <sup>†</sup>    |
+| `attr`              | `foo=""`                   | `draggable=""` <sup>†</sup>       |
+
+<small>†: changed</small>
+
+Coercion for boolean attributes is left untouched.
+
+## Migration Strategy
+
+### Enumerated attributes
+
+The absence of an enumerated attribute and `attr="false"` may produce different IDL attribute values (which will reflect the actual state), described as follows:
+
+| Absent enumerated attr | IDL attr & value                     |
+| ---------------------- | ------------------------------------ |
+| `contenteditable`      | `contentEditable` &rarr; `'inherit'` |
+| `draggable`            | `draggable` &rarr; `false`           |
+| `spellcheck`           | `spellcheck` &rarr; `true`           |
+
+To keep the old behavior work, and as we will be coercing `false` to `'false'`, in 3.x Vue developers need to make `v-bind` expression resolve to `false` or `'false'` for `contenteditable` and `spellcheck`.
+
+In 2.x, invalid values were coerced to `'true'` for enumerated attributes. This was usually unintended and unlikely to be relied upon on a large scale. In 3.x `true` or `'true'` should be explicitly specified.
+
+### Coercing `false` to `'false'` instead of removing the attribute
+
+In 3.x, `null` or `undefined` should be used to explicitly remove an attribute.
+
+### Comparison between 2.x & 3.x behavior
+
+<table>
+  <thead>
+    <tr>
+      <th>Attribute</th>
+      <th><code>v-bind</code> value <sup>2.x</sup></th>
+      <th><code>v-bind</code> value <sup>3.x</sup></th>
+      <th>HTML output</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td rowspan="3">2.x “Enumerated attrs”<br><small>i.e. <code>contenteditable</code>, <code>draggable</code> and <code>spellcheck</code>.</small></td>
+      <td><code>undefined</code>, <code>false</code></td>
+      <td><code>undefined</code>, <code>null</code></td>
+      <td><i>removed</i></td>
+    </tr>
+    <tr>
+      <td>
+        <code>true</code>, <code>'true'</code>, <code>''</code>, <code>1</code>,
+        <code>'foo'</code>
+      </td>
+      <td><code>true</code>, <code>'true'</code></td>
+      <td><code>"true"</code></td>
+    </tr>
+    <tr>
+      <td><code>null</code>, <code>'false'</code></td>
+      <td><code>false</code>, <code>'false'</code></td>
+      <td><code>"false"</code></td>
+    </tr>
+    <tr>
+      <td rowspan="2">Other non-boolean attrs<br><small>eg. <code>aria-checked</code>, <code>tabindex</code>, <code>alt</code>, etc.</small></td>
+      <td><code>undefined</code>, <code>null</code>, <code>false</code></td>
+      <td><code>undefined</code>, <code>null</code></td>
+      <td><i>removed</i></td>
+    </tr>
+    <tr>
+      <td><code>'false'</code></td>
+      <td><code>false</code>, <code>'false'</code></td>
+      <td><code>"false"</code></td>
+    </tr>
+  </tbody>
+</table>