CSS: ADD ::first-line pseudo-element document (#21291)

* CSS: ADD ::first-line pseudo-element document

* fix links, they were mistitled

guide/english/css/selectors/pseudo/first-line/index.md

@@ -0,0 +1,28 @@
+---
+title: First-Line
+---
+## First-Line
+
+The ```::first-line``` CSS pseudo-element styles the first line of a selected HTML element.
+
+General Syntax:
+
+```css
+::first-line
+```
+
+## Example
+
+```css
+/* "First line of every paragraph is set to bold*/
+p::first-line {
+  font-weight: bold;
+}
+```
+
+The ```::first-line``` CSS pseudo-element is often paired with the ```::first-letter``` CSS pseudo-element for a drop caps effect emphasizing the first letter and line of a paragraph.
+
+#### More Information:
+* [MDN Web Docs](https://developer.mozilla.org/en-US/docs/Web/CSS/::first-line)
+* [W3 Schools](https://www.w3schools.com/cssref/sel_firstline.asp)
+* [CSS Tricks](https://css-tricks.com/almanac/selectors/f/first-line/)