Implement a in body

Adoption agency will be handled later

lib/ActiveFormattingElementsList.php

@@ -37,7 +37,7 @@ class ActiveFormattingElementsList extends Stack {
         ), new \Exception("Active formatting element value is invalid"));
         if ($value instanceof ActiveFormattingElementsMarker) {
             $this->_storage[$offset] = $value;
-        } elseif (($offset ?? $count) === $count) {
+        } elseif ($count && ($offset ?? $count) === $count) {
             # When the steps below require the UA to push onto the list of active formatting
             # elements an element element, the UA must perform the following steps:
             // First find the position of the last marker, if any
@@ -60,12 +60,12 @@ class ActiveFormattingElementsList extends Stack {
                 // Stop once there are three matches or the marker is reached 
             } while ($matches < 3 && (--$pos) > $lastMarker);
             if ($matches === 3) {
-                array_splice($this->_storage, $pos, 1, []);
+                $this->offsetUnset($pos);
             }
             # Add element to the list of active formatting elements.
             $this->_storage[] = $value;
         } else {
-            $this->_storage[$offset] = $value;
+            $this->_storage[$offset ?? $count] = $value;
         }
     }
 
@@ -178,6 +178,34 @@ class ActiveFormattingElementsList extends Stack {
             }
         }
     }
+
+    public function findSame(Element $target): int {
+        foreach ($this as $k => $entry) {
+            if ($entry instanceof Element && $entry['element']->isSameNode($target)) {
+                return $k;
+            }
+        }
+        return -1;
+    }
+
+    public function findToMarker(string ...$name): int {
+        foreach ($this as $k => $entry) {
+            if ($entry instanceof ActiveFormattingElementsMarker) {
+                return -1;
+            }
+            if (in_array($entry['element']->nodeName, $name)) {
+                return $k;
+            }
+        }
+        return -1;
+    }
+
+    public function removeSame(Element $target): void {
+        $pos = $this->findSame($target);
+        if ($pos > -1) {
+            unset($this[$pos]);
+        }
+    }
 }
 
 class ActiveFormattingElementsMarker {}

lib/OpenElementsStack.php

@@ -126,7 +126,7 @@ class OpenElementsStack extends Stack {
         return -1;
     }
 
-    public function findSame(\DOMElement $target): int {
+    public function findSame(Element $target): int {
         foreach ($this as $k => $node) {
             if ($node->isSameNode($target)) {
                 return $k;
@@ -135,6 +135,13 @@ class OpenElementsStack extends Stack {
         return -1;
     }
 
+    public function removeSame(Element $target): void {
+        $pos = $this->findSame($target);
+        if ($pos > -1) {
+            unset($this[$pos]);
+        }
+    }
+
     public function generateImpliedEndTags(string ...$exclude): void {
         # When the steps below require the UA to generate implied end tags, 
         #   then, while the current node is {elided list of element names},

lib/ParseError.php

@@ -65,6 +65,7 @@ class ParseError {
     const UNEXPECTED_START_TAG                                              = 206;
     const UNEXPECTED_END_TAG                                                = 207; // html5lib also uses 'adoption-agency-1.2' and 'adoption-agency-1.3' for this
     const NON_VOID_HTML_ELEMENT_START_TAG_WITH_TRAILING_SOLIDUS             = 208;
+    const UNEXPECTED_START_TAG_IMPLIES_END_TAG                              = 209;
 
     const MESSAGES = [
         self::EXPECTED_DOCTYPE_BUT_GOT_START_TAG                                => 'Expected DOCTYPE but got start tag',
@@ -75,6 +76,7 @@ class ParseError {
         self::UNEXPECTED_START_TAG                                              => 'Unexpected start tag',
         self::UNEXPECTED_END_TAG                                                => 'Unexpected end tag',
         self::NON_VOID_HTML_ELEMENT_START_TAG_WITH_TRAILING_SOLIDUS             => 'Trailing solidus in non-void HTML element start tag',
+        self::UNEXPECTED_START_TAG_IMPLIES_END_TAG                              => 'Unexpcted non-nesting start tag in nested context',
 
         self::ENCODING_ERROR                                                    => 'Corrupt encoding near byte position %s',
         self::UNEXPECTED_NULL_CHARACTER                                         => 'Unexpected null character',

lib/TreeBuilder.php

@@ -1354,17 +1354,28 @@ class TreeBuilder {
                     # 4. Set the frameset-ok flag to "not ok".
                     $this->framesetOk = false;
                 }
+                # A start tag whose tag name is "a"
                 elseif ($token->name === "a") {
                     # If the list of active formatting elements contains an a element between the end
                     #   of the list and the last marker on the list (or the start of the list if there
                     #   is no marker on the list), then this is a parse error;
-                    $this->error(ParseError::UNEXPECTED_START_TAG);
-                    # ... run the adoption agency algorithm for the token, 
-                    $this->adopt($token);
-                    # ... then remove that element from the list of active formatting elements and the 
-                    #   stack of open elements if the adoption agency algorithm didn't already remove it
-                    #   (it might not have if the element is not in table scope).
-                    throw new NotImplementedException("NOT IMPLEMENTED");
+                    if (($pos = $this->activeFormattingElementsList->findToMarker("a")) > -1) {
+                        $this->error(ParseError::UNEXPECTED_START_TAG_IMPLIES_END_TAG);
+                        $element = $this->activeFormattingElementsList[$pos]['element'];
+                        # ... run the adoption agency algorithm for the token, 
+                        $this->adopt($token);
+                        # ... then remove that element from the list of active formatting elements and the 
+                        #   stack of open elements if the adoption agency algorithm didn't already remove it
+                        #   (it might not have if the element is not in table scope).
+                        $this->activeFormattingElementsList->removeSame($element);
+                        $this->Stack->removeSame($element);
+                    }
+                    # Reconstruct the active formatting elements, if any.
+                    $this->activeFormattingElementsList->reconstruct();
+                    # Insert an HTML element for the token.
+                    $element = $this->insertStartTagToken($token);
+                    # Push onto the list of active formatting elements that element.
+                    $this->activeFormattingElementsList->insert($token, $element);
                 } else {
                     throw new NotImplementedException("NOT IMPLEMENTED");
                 }