Some new features add associated limits. This document records a design policy for such features.

- `{ newLimit: undefined }` is allowed even if `newLimit` is unknown. (It has no effect.)

- Limits entries associated with features have these Adapter Capability Guarantees:

- If the feature is available, its limits are `better` than or equal to the default.

- If not, its limits are `undefined`.

- They are `undefined` if the feature is unavailable on the adapter.

- It is allowed to request a limit that's available on the adapter (that is,

not `undefined` in `adapter.limits`), even without also requesting the corresponding feature.

It won't have any effect, but the limit request will still be validated against the adapter.

- It is allowed to request a feature that's available on the adapter, even without also

requesting the corresponding limit(s). (They will get their default values.)

- There should be a feature name for each such feature - don't just add a new limit

(defaulting to 0) on its own.

- Allows us to clearly guarantee a default value for the limit when the feature is available.

(You only have to request the feature, not figure out what limit you need to request.)

- Is consistent with features that add 0 limits or more than 1 limit.

- Each new limit should define its "default" value as the default non-trivial value

(that is, ignoring whether the feature is available/requested).

The new feature is `"foo"`.

The new limit is `maxFoo`, and it has a default value of `5`.

For each case study we'll look at how a piece of code behaves in several cases given an adapter `a`:

- *Available (5)*: a browser which implements `"foo"`, on a device with support with `maxFoo: 5`.

- `a.features.has("foo")` is `true`

- `'maxFoo' in a.requiredLimits` is `true`

- `a.requiredLimits.maxFoo` is `7`

- *Available (7)*: a browser which implements `"foo"`, on a device with support with `maxFoo: 7`.

- `a.features.has("foo")` is `true`

- `'maxFoo' in a.requiredLimits` is `true`

- `a.requiredLimits.maxFoo` is `7`

- *Unavailable*: a browser which implements `"foo"`, on a device without support.

- `a.features.has("foo")` is `false`

- `'maxFoo' in a.requiredLimits` is `true`

- `a.requiredLimits.maxFoo` is `undefined`

- *Unimplemented*: a browser which hasn't implemented `"foo"` **but does implement the new rules allowing unknown limits set to `undefined`**

- `a.features.has("foo")` is `false`

- `'maxFoo' in a.requiredLimits` is `false`

- `a.requiredLimits.maxFoo` is `undefined`

In all cases `requiredFeatures: []` is the same as not specifying it,

`requiredLimits: {}` is the same as not specifying it, and

`maxFoo: undefined` is the same as not specifying it.

We'll ignore the `requiredLimits: a.limits` case, as it should be equivalent to one of the other cases

(which one depends on the resolution of [#4277](https://github.com/gpuweb/gpuweb/issues/4277)).

- `a.requestDevice({ requiredFeatures: [], requiredLimits: { maxFoo: undefined } })`

- *Available (7)*: OK, request ignored, device limit is `undefined`

- *Available (5)*: OK, request ignored, device limit is `undefined`

- *Unavailable*: OK, request ignored, device limit is `undefined`

- *Unimplemented*: OK, request ignored, device limit is `undefined`

- `a.requestDevice({ requiredFeatures: [], requiredLimits: { maxFoo: a.limits.maxFoo } })`

- *Available (7)*: OK, device limit is `undefined` (request has no effect)

- *Available (5)*: OK, device limit is `undefined` (request has no effect)

- *Unavailable*: OK (limit request ignored because undefined)

- *Unimplemented*: OK (limit request ignored because undefined)

- `a.requestDevice({ requiredFeatures: [], requiredLimits: { maxFoo: 5 } })`

- *Available (7)*: OK, device limit is `undefined` (request has no effect)

- *Available (5)*: OK, device limit is `undefined` (request has no effect)

- *Unavailable*: error, limit key not available

- *Unimplemented*: error, limit key not available

- `a.requestDevice({ requiredFeatures: [], requiredLimits: { maxFoo: 7 } })`

- *Available (7)*: OK, device limit is `undefined` (request has no effect)

- *Available (5)*: error, limit value not available (even though request would have no effect)

- *Unavailable*: error, limit key not available

- *Unimplemented*: error, limit key not available

- `a.requestDevice({ requiredFeatures: ["foo"], requiredLimits: { maxFoo: undefined } })`

- *Available (7)*: OK, enabled with limit set to default (5)

- *Available (5)*: OK, enabled with limit set to default (5)

- *Unavailable*: error, feature not available

- *Unimplemented*: error, feature not available

- `a.requestDevice({ requiredFeatures: ["foo"], requiredLimits: { maxFoo: a.limits.maxFoo } })`

- *Available (7)*: OK, enabled with limit set to 7

- *Available (5)*: OK, enabled with limit set to 5

- *Unavailable*: error, feature not available

- *Unimplemented*: error, feature not available

- `a.requestDevice({ requiredFeatures: ["foo"], requiredLimits: { maxFoo: 5 } })`

- *Available (7)*: OK, enabled with limit set to 5

- *Available (5)*: OK, enabled with limit set to 5

- *Unavailable*: error, feature not available, limit key not available

- *Unimplemented*: error, feature not available, limit key not available

- `a.requestDevice({ requiredFeatures: ["foo"], requiredLimits: { maxFoo: 7 } })`

- *Available (7)*: OK, enabled with limit set to 7

- *Available (5)*: error, limit value not available

- *Unavailable*: error, feature not available, limit key not available

- *Unimplemented*: error, feature not available, limit key not available

- `a.requestDevice({ requiredFeatures: a.features, requiredLimits: { maxFoo: undefined } })`

- *Available (7)*: OK, enabled with limit set to default (5)

- *Available (5)*: OK, enabled with limit set to default (5)

- *Unavailable*: OK, neither limit nor feature is requested

- *Unimplemented*: OK, neither limit nor feature is requested

- `a.requestDevice({ requiredFeatures: a.features, requiredLimits: { maxFoo: a.limits.maxFoo } })`

- *Available (7)*: OK, enabled with limit set to 7

- *Available (5)*: OK, enabled with limit set to 5

- *Unavailable*: OK, neither limit nor feature is requested

- *Unimplemented*: OK, neither limit nor feature is requested

- `a.requestDevice({ requiredFeatures: a.features, requiredLimits: { maxFoo: 5 } })`

- *Available (7)*: OK, enabled with limit set to 5

- *Available (5)*: OK, enabled with limit set to 5

- *Unavailable*: error, limit key not available

- *Unimplemented*: error, limit key not available

- `a.requestDevice({ requiredFeatures: a.features, requiredLimits: { maxFoo: 7 } })`

- *Available (7)*: OK, enabled with limit set to 7

- *Available (5)*: error, limit value not available

- *Unavailable*: error, limit key not available

- *Unimplemented*: error, limit key not available