Add pseudo element constants and docs

styles/STYLEMANAGER.md

@@ -10,6 +10,7 @@
     - [Creating a StyleManager](#creating-a-stylemanager)
     - [Adding Styles](#adding-styles)
     - [Pseudo-Classes](#pseudo-classes)
+    - [Pseudo-Elements](#pseudo-elements)
     - [Animations](#animations)
     - [Media Queries](#media-queries)
 - [Features](#features)
@@ -102,6 +103,25 @@ link := elem.A(
 )
 ```
 
+### Pseudo-Elements
+
+Define styles with pseudo-elements to insert content or style specific parts of an element:
+
+```go
+compositeClassName := styleMgr.AddCompositeStyle(styles.CompositeStyle{
+    Default: styles.Props{
+		styles.Position: "relative"
+	},
+    PseudoElements: map[string]styles.Props{
+        styles.PseudoBefore: {
+			styles.Content: "'Before Text'"
+		},
+    },
+})
+```
+
+In this example, the `::before` pseudo-element will insert the text "Before Text" before the content of the element.
+
 ### Animations
 
 Create animations and apply them to styles with the returned animation name:

styles/constants.go

@@ -214,6 +214,21 @@ const (
 	PseudoPaused  = ":paused"
 
 	// Content pseudo-elements
-	PseudoBefore = "::before"
-	PseudoAfter = "::after"
+	PseudoBefore              = "::before"
+	PseudoAfter               = "::after"
+	PseudoFirstLine           = "::first-line"
+	PseudoFirstLetter         = "::first-letter"
+	PseudoSelection           = "::selection"
+	PseudoBackdrop            = "::backdrop"
+	PseudoPlaceholder         = "::placeholder"
+	PseudoMarker              = "::marker"
+	PseudoCues                = "::cues"
+	PseudoCueRegion           = "::cue-region"
+	PseudoResizer             = "::resizer"
+	PseudoScrollbar           = "::scrollbar"
+	PseudoScrollbarThumb      = "::scrollbar-thumb"
+	PseudoScrollbarTrack      = "::scrollbar-track"
+	PseudoScrollbarTrackPiece = "::scrollbar-track-piece"
+	PseudoScrollbarButton     = "::scrollbar-button"
+	PseudoScrollbarCorner     = "::scrollbar-corner"
 )