Update index.md - Add information on the rcap, rex, ric, and `r…

…ch` units (#31901)

* Update index.md - add information on `rcap`, `rex`, `ric`, and `rch` units

I am adding information about the `rcap`, `rex`, `ric`, and `rch` units. Issue #509.

* Update files/en-us/web/css/length/index.md

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* Update files/en-us/web/css/length/index.md

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* Update files/en-us/web/css/length/index.md

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* Update files/en-us/web/css/length/index.md

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

* Update files/en-us/web/css/length/index.md

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

* Update files/en-us/web/css/length/index.md

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

* Update files/en-us/web/css/length/index.md

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

* Update files/en-us/web/css/length/index.md

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

* Update files/en-us/web/css/length/index.md

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

* Update files/en-us/web/css/length/index.md

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* Update files/en-us/web/css/length/index.md

Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Dipika Bhattacharya <dipika@foss-community.org>

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

@@ -43,11 +43,22 @@ Font lengths define the `<length>` value in terms of the size of a particular ch
 - `ic`
   - : Equal to the used {{Glossary("advance measure")}} of the "水" glyph (CJK water ideograph, U+6C34), found in the font used to render it.
 - `lh`
-  - : Equal to the computed value of the {{Cssxref("line-height")}} property of the element on which it is used, converted to an absolute length.
+  - : Equal to the computed value of the {{Cssxref("line-height")}} property of the element on which it is used, converted to an absolute length. This unit enables length calculations based on the theoretical size of an ideal empty line. However, the size of actual line boxes may differ based on their content.
+
+### Relative length units based on root element's font
+
+- `rcap`
+  - : Equal to the "cap height" (nominal height of capital letters) of the root element's {{Cssxref("font")}}.
+- `rch`
+  - : Equal to the width or the {{Glossary("advance measure")}} of the glyph `0` (zero, the Unicode character U+0030) in the root element's {{Cssxref("font")}}.
 - `rem`
-  - : Represents the {{Cssxref("font-size")}} of the root element (typically {{HTMLElement("html")}}). When used within the root element {{Cssxref("font-size")}}, it represents its initial value (a common browser default is `16px`, but user-defined preferences may modify this).
+  - : Represents the {{Cssxref("font-size")}} of the root element (typically {{HTMLElement("html")}}). When used within the root element {{Cssxref("font-size")}}, it represents its initial value. A common browser default is `16px`, but user-defined preferences may modify this.
+- `rex`
+  - : Represents the x-height of the root element's {{Cssxref("font")}}.
+- `ric`
+  - : Equal to the value of [`ic`](#ic) unit on the root element's font.
 - `rlh`
-  - : Equal to the computed value of the {{Cssxref("line-height")}} property on the root element (typically {{HTMLElement("html")}}), converted to an absolute length. When used on the {{Cssxref("font-size")}} or {{Cssxref("line-height")}} properties of the root element, it refers to the properties' initial value.
+  - : Equal to the value of [`lh`](#lh) unit on the root element's font. This unit enables length calculations based on the theoretical size of an ideal empty line. However, the size of actual line boxes may differ based on their content.
 
 ### Relative length units based on viewport