docs: improve accessibility for custom form-field control guide

Improves the accessibility for the control that is built as part
of the custom form-field control guide.

guides/creating-a-custom-form-field-control.md

@@ -23,7 +23,7 @@ class MyTel {
 @Component({
   selector: 'example-tel-input',
   template: `
-    <div [formGroup]="parts">
+    <div role="group" [formGroup]="parts">
       <input class="area" formControlName="area" maxlength="3">
       <span>&ndash;</span>
       <input class="exchange" formControlName="exchange" maxlength="3">
@@ -45,7 +45,7 @@ class MyTel {
     }
   `],
 })
-class MyTelInput {
+export class MyTelInput {
   parts: FormGroup;
 
   @Input()
@@ -85,7 +85,7 @@ a provider to our component so that the form field will be able to inject it as
   ...
   providers: [{provide: MatFormFieldControl, useExisting: MyTelInput}],
 })
-class MyTelInput implements MatFormFieldControl<MyTel> {
+export class MyTelInput implements MatFormFieldControl<MyTel> {
   ...
 }
 ```
@@ -201,7 +201,7 @@ To resolve this, remove the `NG_VALUE_ACCESSOR` provider and instead set the val
     // },
   ],
 })
-class MyTelInput implements MatFormFieldControl<MyTel> {
+export class MyTelInput implements MatFormFieldControl<MyTel> {
   constructor(
     ...,
     @Optional() @Self() public ngControl: NgControl,
@@ -341,16 +341,20 @@ controlType = 'example-tel-input';
 
 This method is used by the `<mat-form-field>` to specify the IDs that should be used for the
 `aria-describedby` attribute of your component. The method has one parameter, the list of IDs, we
-just need to apply the given IDs to our host element.
+just need to apply the given IDs the element that represents our control. 
 
-```ts
-@HostBinding('attr.aria-describedby') describedBy = '';
+In our concrete example, these IDs would need to be applied to the group element. 
 
+```ts
 setDescribedByIds(ids: string[]) {
   this.describedBy = ids.join(' ');
 }
 ```
 
+```html
+<div role="group" [formGroup]="parts" [attr.aria-describedby]="describedBy">
+```
+
 #### `onContainerClick(event: MouseEvent)`
 
 This method will be called when the form field is clicked on. It allows your component to hook in
@@ -366,6 +370,41 @@ onContainerClick(event: MouseEvent) {
 }
 ```
 
+### Improving accessibility
+
+Our custom form field control consists of multiple inputs that describe segments of a phone
+number. For accessibility purposes, we put those inputs as part of a `div` element with
+`role="group"`. This ensures that screen reader users can tell that all those inputs belong
+together.
+
+One significant information is missing for screen reader users though. They won't be able
+to tell what this input group represents. To improve this, we should add a label for the group
+element using either `aria-label` or `aria-labelledby`.
+
+It's recommended to link the group to the label that is displayed as part of the parent
+`<mat-form-field>`. This ensures that explicitly specified labels (using `<mat-label>`) are
+actually used for labelling the control.
+
+In our concrete example, we add an attribute binding for `aria-labelledby` and bind it
+to the label element id provided by the parent `<mat-form-field>`.
+
+```typescript
+export class MyTelInput implements MatFormFieldControl<MyTel> {
+  ...
+
+  constructor(...
+              @Optional() public parentFormField: MatFormField) {
+```
+
+```html
+@Component({
+  selector: 'example-tel-input',
+  template: `
+    <div role="group" [formGroup]="parts"
+         [attr.aria-describedby]="describedBy"
+         [attr.aria-labelledby]="parentFormField?.getLabelId()">
+```
+
 ### Trying it out
 
 Now that we've fully implemented the interface, we're ready to try our component out! All we need to

src/components-examples/material-experimental/mdc-form-field/mdc-form-field-custom-control/example-tel-input-example.html

@@ -1,4 +1,6 @@
-<div [formGroup]="parts" class="example-tel-input-container">
+<div role="group" class="example-tel-input-container" [formGroup]="parts"
+     [attr.aria-labelledby]="_formField?.getLabelId()"
+     [attr.aria-describedby]="describedBy">
   <input class="example-tel-input-element" formControlName="area" size="3"
          aria-label="Area code" (input)="_handleInput()">
   <span class="example-tel-input-spacer">&ndash;</span>

src/components-examples/material-experimental/mdc-form-field/mdc-form-field-custom-control/form-field-custom-control-example.ts

@@ -2,7 +2,7 @@ import {FocusMonitor} from '@angular/cdk/a11y';
 import {coerceBooleanProperty} from '@angular/cdk/coercion';
 import {Component, ElementRef, Input, OnDestroy, Optional, Self} from '@angular/core';
 import {FormBuilder, FormGroup, ControlValueAccessor, NgControl} from '@angular/forms';
-import {MatFormFieldControl} from '@angular/material-experimental/mdc-form-field';
+import {MatFormField, MatFormFieldControl} from '@angular/material-experimental/mdc-form-field';
 import {Subject} from 'rxjs';
 
 /** @title Form field with custom telephone number input control. */
@@ -27,7 +27,6 @@ export class MyTel {
   host: {
     '[class.example-floating]': 'shouldLabelFloat',
     '[id]': 'id',
-    '[attr.aria-describedby]': 'describedBy',
   }
 })
 export class MyTelInput implements ControlValueAccessor, MatFormFieldControl<MyTel>, OnDestroy {
@@ -94,6 +93,7 @@ export class MyTelInput implements ControlValueAccessor, MatFormFieldControl<MyT
     formBuilder: FormBuilder,
     private _focusMonitor: FocusMonitor,
     private _elementRef: ElementRef<HTMLElement>,
+    @Optional() public _formField: MatFormField,
     @Optional() @Self() public ngControl: NgControl) {
 
     this.parts = formBuilder.group({

src/components-examples/material/form-field/form-field-custom-control/example-tel-input-example.html

@@ -1,4 +1,6 @@
-<div [formGroup]="parts" class="example-tel-input-container">
+<div role="group" class="example-tel-input-container" [formGroup]="parts"
+     [attr.aria-labelledby]="_formField?.getLabelId()"
+     [attr.aria-describedby]="describedBy">
   <input class="example-tel-input-element" formControlName="area" size="3" aria-label="Area code" (input)="_handleInput()">
   <span class="example-tel-input-spacer">&ndash;</span>
   <input class="example-tel-input-element" formControlName="exchange" size="3" aria-label="Exchange code" (input)="_handleInput()">

src/components-examples/material/form-field/form-field-custom-control/form-field-custom-control-example.ts

@@ -2,7 +2,7 @@ import {FocusMonitor} from '@angular/cdk/a11y';
 import {coerceBooleanProperty} from '@angular/cdk/coercion';
 import {Component, ElementRef, Input, OnDestroy, Optional, Self} from '@angular/core';
 import {FormBuilder, FormGroup, ControlValueAccessor, NgControl} from '@angular/forms';
-import {MatFormFieldControl} from '@angular/material/form-field';
+import {MatFormField, MatFormFieldControl} from '@angular/material/form-field';
 import {Subject} from 'rxjs';
 
 /** @title Form field with custom telephone number input control. */
@@ -27,7 +27,6 @@ export class MyTel {
   host: {
     '[class.example-floating]': 'shouldLabelFloat',
     '[id]': 'id',
-    '[attr.aria-describedby]': 'describedBy',
   }
 })
 export class MyTelInput implements ControlValueAccessor, MatFormFieldControl<MyTel>, OnDestroy {
@@ -94,6 +93,7 @@ export class MyTelInput implements ControlValueAccessor, MatFormFieldControl<MyT
     formBuilder: FormBuilder,
     private _focusMonitor: FocusMonitor,
     private _elementRef: ElementRef<HTMLElement>,
+    @Optional() public _formField: MatFormField,
     @Optional() @Self() public ngControl: NgControl) {
 
     this.parts = formBuilder.group({