Revert freeform dimension units

Author: drwpow

technical-reports/format/types.md

@@ -51,12 +51,12 @@ $translucent-shadow: hsla(300, 100%, 50%, 0.5);
 
 ## Dimension
 
-Represents an amount of distance in a single dimension in the UI, such as a position, width, height, radius, or thickness. The `$type` property MUST be set to the string `dimension`. The value MUST be an object with `value` and `unit` keys.
+Represents an amount of distance in a single dimension in the UI, such as a position, width, height, radius, or thickness. The `$type` property MUST be set to the string `dimension`. The value MUST be an object containing a numeric `value` (integer or floating-point) and `unit` of measurement (`"px"` or `"rem"`).
 
 | Key     |   Type   | Required | Description                                                        |
 | :------ | :------: | :------: | :----------------------------------------------------------------- |
 | `value` | `number` |    Y     | An integer or floating-point value representing the numeric value. |
-| `unit`  | `string` |    Y     | A string specifying the units of measurement                       |
+| `unit`  | `string` |    Y     | Unit of distance. Supported values: `"px"`, `"rem"`.               |
 
 For example:
 
@@ -85,8 +85,9 @@ For example:
 
 ### Validation
 
-- `$value.unit` may be any string. It is up to tooling to translate units into the appropriate platform.
-- `$value.unit` may NOT be an empty string (`""`).
+- `$value.unit` may only be `"px"` or `"rem"`.
+  - **px**: Represents an idealized pixel on the viewport. The equivalent in Android is `dp` and iOS is `pt`. Translation tools SHOULD therefore convert to these or other equivalent units as needed.
+  - **rem**: Represents a multiple of the system's default font size (which MAY be configurable by the user). 1rem is 100% of the default font size. The equivalent of 1rem on Android is 16sp. Not all platforms have an equivalent to rem, so translation tools MAY need to do a lossy conversion to a fixed px size by assuming a default font size (usually 16px) for such platforms.
 - `$value.unit` is still requird even if `$value.value` is `0`.
 
 ## Font family
@@ -160,12 +161,12 @@ Example:
 
 ## Duration
 
-Represents the length of time in milliseconds an animation or animation cycle takes to complete, such as 200 milliseconds. The `$type` property MUST be set to the string `duration`. The value MUST be an object containin a numeric `value` (either integer or floating-point) and optionally a `unit` to specify milliseconds (`"ms"`) or seconds (`"s"`). A millisecond is a unit of time equal to one thousandth of a second.
+Represents the length of time in milliseconds an animation or animation cycle takes to complete, such as 200 milliseconds. The `$type` property MUST be set to the string `duration`. The value MUST be an object containing a numeric `value` (either integer or floating-point) and a `unit` of milliseconds (`"ms"`) or seconds (`"s"`). A millisecond is a unit of time equal to one thousandth of a second.
 
-| Key     |   Type   | Required | Description                                                            |
-| :------ | :------: | :------: | :--------------------------------------------------------------------- |
-| `value` | `number` |    Y     | An integer or floating-point value representing the numeric value.     |
-| `unit`  | `string` |          | The unit of time. Only `"ms"`, `"s`", or (undefined) are valid values. |
+| Key     |   Type   | Required | Description                                                          |
+| :------ | :------: | :------: | :------------------------------------------------------------------- |
+| `value` | `number` |    Y     | An integer or floating-point value representing the numeric value.   |
+| `unit`  | `string` |    Y     | Unit of time. Supported values: `"ms"` (millisecond), `"s"`(second). |
 
 For example: