diff --git a/lib/ActiveFormattingElementsList.php b/lib/ActiveFormattingElementsList.php
index f666dda..bd9c6b0 100644
--- a/lib/ActiveFormattingElementsList.php
+++ b/lib/ActiveFormattingElementsList.php
@@ -25,11 +25,11 @@ class ActiveFormattingElementsList extends Stack {
     }
 
     public function offsetSet($offset, $value) {
-        $count = count($this->_storage);
+        $count = $this->count;
         assert($offset >= 0 && $offset <= $count, new Exception(Exception::STACK_INVALID_INDEX, $offset));
         assert($value instanceof ActiveFormattingElementsMarker || (
             is_array($value) 
-            && sizeof($value) === 2 
+            && count($value) === 2 
             && isset($value['token']) 
             && isset($value['element'])
             && $value['token'] instanceof StartTagToken
@@ -69,6 +69,7 @@ class ActiveFormattingElementsList extends Stack {
         } else {
             $this->_storage[$offset ?? $count] = $value;
         }
+        $this->count = count($this->_storage);
     }
 
     protected function matchElement(Element $a, Element $b): bool {
@@ -99,7 +100,7 @@ class ActiveFormattingElementsList extends Stack {
     }
 
     public function insert(StartTagToken $token, Element $element, ?int $at = null): void  {
-        assert($at === null || ($at >= 0 && $at <= count($this->_storage)), new \Exception("Invalid list index $at (max ".count($this->_storage).")"));
+        assert($at === null || ($at >= 0 && $at <= $this->count), new \Exception("Invalid list index $at (max ".$this->count.")"));
         if ($at === null) {
             $this[] = [
                 'token' => $token,
@@ -110,6 +111,7 @@ class ActiveFormattingElementsList extends Stack {
                 'token' => $token,
                 'element' => $element,
             ]]);
+            $this->count = count($this->_storage);
         }
     }
 
@@ -125,7 +127,7 @@ class ActiveFormattingElementsList extends Stack {
         if (!$this->_storage) {
             return;
         }
-        $last = count($this->_storage) - 1;
+        $last = $this->count - 1;
         # 2. If the last (most recently added) entry in the list of active formatting
         #   elements is a marker, or if it is an element that is in the stack of open
         #   elements, then there is nothing to reconstruct; stop this algorithm.
@@ -187,6 +189,7 @@ class ActiveFormattingElementsList extends Stack {
                 break;
             }
         }
+        $this->count = count($this->_storage);
     }
 
     public function findSame(Element $target): int {
diff --git a/lib/OpenElementsStack.php b/lib/OpenElementsStack.php
index 6cf5003..8b6235b 100644
--- a/lib/OpenElementsStack.php
+++ b/lib/OpenElementsStack.php
@@ -138,8 +138,8 @@ class OpenElementsStack extends Stack {
             $this[] = $element;
         } else {
             array_splice($this->_storage, $at, 0, [$element]);
-            $this->computeProperties();
         }
+        $this->computeProperties();
     }
 
     public function popUntil(string ...$target): void {
@@ -205,6 +205,7 @@ class OpenElementsStack extends Stack {
         }
         while (!$this->isEmpty() && $this->top()->namespaceURI === null && ($map[$this->top()->nodeName] ?? false)) {
             array_pop($this->_storage);
+            $this->count--;
         }
         $this->computeProperties();
     }
@@ -215,6 +216,7 @@ class OpenElementsStack extends Stack {
         #   the UA must pop the current node off the stack of open elements.
         while (!$this->isEmpty() && $this->top()->namespaceURI === null && (self::IMPLIED_END_TAGS_THOROUGH[$this->top()->nodeName] ?? false)) {
             array_pop($this->_storage);
+            $this->count--;
         }
         $this->computeProperties();
     }
@@ -224,11 +226,11 @@ class OpenElementsStack extends Stack {
         #   table context, it means that the UA must, while the current node
         #   is not a table, template, or html element, pop elements from the
         #   stack of open elements.
-        assert(sizeof($this->_storage) > 0, new \Exception("Stack is empty"));
+        assert(count($this->_storage) > 0, new \Exception("Stack is empty"));
         $pos = $this->find("table", "template", "html");
         assert($pos > -1, new \Exception("No table context exists"));
         $stop = $pos + 1;
-        while (sizeof($this->_storage) > $stop) {
+        while (count($this->_storage) > $stop) {
             array_pop($this->_storage);
         }
         $this->computeProperties();
@@ -239,11 +241,11 @@ class OpenElementsStack extends Stack {
         #   table body context, it means that the UA must, while the current
         #   node is not a tbody, tfoot, thead, template, or html element,
         #   pop elements from the stack of open elements.
-        assert(sizeof($this->_storage) > 0, new \Exception("Stack is empty"));
+        assert(count($this->_storage) > 0, new \Exception("Stack is empty"));
         $pos = $this->find("tbody", "tfoot", "thead", "template", "html");
         assert($pos > -1, new \Exception("No table body context exists"));
         $stop = $pos + 1;
-        while (sizeof($this->_storage) > $stop) {
+        while (count($this->_storage) > $stop) {
             array_pop($this->_storage);
         }
         $this->computeProperties();
@@ -254,11 +256,11 @@ class OpenElementsStack extends Stack {
         #   table row context, it means that the UA must, while the current
         #   node is not a tr, template, or html element, pop elements from
         #   the stack of open elements.
-        assert(sizeof($this->_storage) > 0, new \Exception("Stack is empty"));
+        assert(count($this->_storage) > 0, new \Exception("Stack is empty"));
         $pos = $this->find("tr", "template", "html");
         assert($pos > -1, new \Exception("No table row context exists"));
         $stop = $pos + 1;
-        while (sizeof($this->_storage) > $stop) {
+        while (count($this->_storage) > $stop) {
             array_pop($this->_storage);
         }
         $this->computeProperties();
@@ -331,12 +333,13 @@ class OpenElementsStack extends Stack {
     }
 
     protected function computeProperties(): void {
+        $this->count = count($this->_storage);
         $this->currentNode = $this->top();
         # The adjusted current node is the context element if the parser was created by
         # the HTML fragment parsing algorithm and the stack of open elements has only one
         # element in it (fragment case); otherwise, the adjusted current node is the
         # current node.
-        if ($this->fragmentContext && sizeof($this->_storage) === 1) {
+        if ($this->fragmentContext && $this->count === 1) {
             $this->adjustedCurrentNode = $this->fragmentContext;
         } else {
             $this->adjustedCurrentNode = $this->currentNode;
diff --git a/lib/Parser.php b/lib/Parser.php
index 13f918d..5f8428e 100644
--- a/lib/Parser.php
+++ b/lib/Parser.php
@@ -74,7 +74,7 @@ class Parser {
                 // Try to find a Content-Type header-field
                 foreach ($meta['wrapper_data'] as $h) {
                     $h = explode(":", $h, 2);
-                    if (sizeof($h) === 2) {
+                    if (count($h) === 2) {
                         if (preg_match("/^\s*Content-Type\s*$/i", $h[0])) {
                             // Try to get an encoding from it
                             $encoding = Charset::fromTransport($h[1]);
diff --git a/lib/Stack.php b/lib/Stack.php
index f4b6e7b..6bdcfc6 100644
--- a/lib/Stack.php
+++ b/lib/Stack.php
@@ -4,6 +4,7 @@ namespace dW\HTML5;
 
 abstract class Stack implements \ArrayAccess, \Countable, \IteratorAggregate {
     protected $_storage = [];
+    protected $count = 0;
 
     public function offsetSet($offset, $value) {
         assert($offset >= 0, new Exception(Exception::STACK_INVALID_INDEX, $offset));
@@ -13,6 +14,7 @@ abstract class Stack implements \ArrayAccess, \Countable, \IteratorAggregate {
         } else {
             $this->_storage[$offset] = $value;
         }
+        $this->count = count($this->_storage);
     }
 
     public function offsetExists($offset) {
@@ -22,6 +24,7 @@ abstract class Stack implements \ArrayAccess, \Countable, \IteratorAggregate {
     public function offsetUnset($offset) {
         assert($offset >= 0 && $offset < count($this->_storage), new Exception(Exception::STACK_INVALID_INDEX, $offset));
         array_splice($this->_storage, $offset, 1, []);
+        $this->count = count($this->_storage);
     }
 
     public function offsetGet($offset) {
@@ -30,16 +33,17 @@ abstract class Stack implements \ArrayAccess, \Countable, \IteratorAggregate {
     }
     
     public function count(): int {
-        return count($this->_storage);
+        return $this->count;
     }
 
     public function getIterator(): \Traversable {
-        for ($a = count($this->_storage) - 1; $a > -1; $a--) {
+        for ($a = $this->count - 1; $a > -1; $a--) {
             yield $a => $this->_storage[$a];
         }
     }
 
     public function pop() {
+        $this->count = max($this->count - 1, 0);
         return array_pop($this->_storage);
     }
 
@@ -49,6 +53,6 @@ abstract class Stack implements \ArrayAccess, \Countable, \IteratorAggregate {
 
     public function top(int $offset = 0) {
         assert($offset >= 0, new \Exception("Offset must be at least 0"));
-        return ($c = count($this->_storage)) > $offset ? $this->_storage[$c - ($offset + 1)] : null;
+        return ($c = $this->count) > $offset ? $this->_storage[$c - ($offset + 1)] : null;
     }
 }