rem() - New CSS functional notation (#23183)

* `rem()` - New CSS functional notation

* Fix lint issues

* Update code examples

* Add more code examples covering additional use cases

* Update index.md

* Return value

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Co-authored-by: StyleShit <32631382+StyleShit@users.noreply.github.com>

* Update index.md

* Update index.md

* Apply suggestions from code review

* Update index.md

* Apply suggestions from code review

* Apply suggestions from code review

Co-authored-by: Estelle Weyl <estelle@openwebdocs.org>

* Update index.md

* Add examples with calculations

* lint

Co-authored-by: StyleShit <32631382+StyleShit@users.noreply.github.com>
Co-authored-by: Estelle Weyl <estelle@openwebdocs.org>

files/en-us/web/css/rem/index.md

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+---
+title: rem()
+slug: Web/CSS/rem
+page-type: css-function
+tags:
+  - CSS
+  - CSS Function
+  - Function
+  - Math
+  - Reference
+  - Web
+  - rem
+  - Experimental
+browser-compat: css.types.rem
+---
+
+{{CSSRef}}{{SeeCompatTable}}
+
+The **`rem()`** [CSS](/en-US/docs/Web/CSS) [function](/en-US/docs/Web/CSS/CSS_Functions) returns a remainder left over when the first parameter is divided by the second parameter, similar to the JavaScript [remainder operator (`%`)](/en-US/docs/Web/JavaScript/Reference/Operators/Remainder). The remainder is the value left over when one operand, the dividend, is divided by a second operand, the divisor. It always takes the sign of the dividend.
+
+> For example, the CSS `rem(27, 5)` function returns the remainder of `2`. When dividing 27 by 5, the result is 5 with a remainder of 2. The full calculation is `27 / 5 = 5 * 5 + 2`.
+
+## Syntax
+
+```css
+/* Unitless <number> */
+line-height: rem(21, 2);  /* 1 */
+line-height: rem(14, 5);  /* 4 */
+line-height: rem(5.5, 2); /* 1.5 */
+
+/* Unit based <percentage> and <dimension> */
+margin: rem(14%, 3%);       /* 2% */
+margin: rem(18px, 5px);     /* 3px */
+margin: rem(10rem, 6rem);   /* 4rem */
+margin: rem(26vmin, 7vmin); /* 5vmin */
+margin: rem(1000px, 29rem); /* 72px - if root font-size is 16px */
+
+/* Negative/positive values */
+rotate: rem(200deg, 30deg);  /* 20deg */
+rotate: rem(140deg, -90deg); /* 50deg */
+rotate: rem(-90deg, 20deg);  /* -10deg */
+rotate: rem(-55deg, -15deg); /* -10deg */
+
+/* Calculations */
+scale: rem(10 * 2, 1.7);         /* 1.3 */
+rotate: rem(10turn, 18turn / 3); /* 4turn */
+transition-duration: rem(20s / 2, 3000ms * 2); /* 4s */
+```
+
+### Parameter
+
+The `rem(dividend, divisor)` function accepts two comma-separated values as its parameters. Both parameters must have the same type, [number](/en-US/docs/Web/CSS/number), [dimension](/en-US/docs/Web/CSS/dimension), or [percentage](/en-US/docs/Web/CSS/percentage), for the function to be valid. While the units in the two parameters don't need to be the same, they do need of the same dimension type, such as {{cssxref("length")}}, {{cssxref("angle")}}, {{cssxref("time")}}, or {{cssxref("frequency")}} to be valid.
+
+- `dividend`
+  - : A calculation that resolves to a {{CSSxREF("&lt;number&gt;")}}, {{CSSxREF("&lt;dimension&gt;")}}, or {{CSSxREF("&lt;percentage&gt;")}} representing the dividend.
+
+- `divisor`
+  - : A calculation that resolves to a {{CSSxREF("&lt;number&gt;")}}, {{CSSxREF("&lt;dimension&gt;")}}, or {{CSSxREF("&lt;percentage&gt;")}} representing the divisor.
+
+### Return value
+
+Returns a {{CSSxREF("&lt;number&gt;")}}, {{CSSxREF("&lt;dimension&gt;")}}, or {{CSSxREF("&lt;percentage&gt;")}} (corresponds to the parameters' type) representing the remainder, that is, the operation left over.
+
+- If `divisor` is `0`, the result is `NaN`.
+- If `dividend` is `infinite`, the result is `NaN`.
+- If `dividend` is positive the result is positive (`0⁺`), and if `dividend` is negative the result is negative (`0⁻`).
+
+### Formal syntax
+
+{{CSSSyntax}}
+
+## Specifications
+
+{{Specifications}}
+
+## Browser compatibility
+
+{{Compat}}
+
+## See also
+
+- {{CSSxRef("round")}}
+- {{CSSxRef("mod")}}