diff --git a/config/php-cs-fixer.php b/config/php-cs-fixer.php
index bc449156..26dfd245 100644
--- a/config/php-cs-fixer.php
+++ b/config/php-cs-fixer.php
@@ -22,6 +22,9 @@
             // overwrite the PER2 defaults to restore compatibility with PHP 7.x
             'trailing_comma_in_multiline' => ['elements' => ['arrays', 'match']],
 
+            // for `in_array` & friends
+            'strict_param' => true,
+
             'php_unit_construct' => true,
             'php_unit_dedicate_assert' => ['target' => 'newest'],
             'php_unit_expectation' => ['target' => 'newest'],
diff --git a/src/Parsing/ParserState.php b/src/Parsing/ParserState.php
index 02c47719..b221acf0 100644
--- a/src/Parsing/ParserState.php
+++ b/src/Parsing/ParserState.php
@@ -372,7 +372,7 @@ public function consumeUntil($aEnd, $bIncludeEnd = false, $consumeEnd = false, a
 
         while (!$this->isEnd()) {
             $char = $this->consume(1);
-            if (in_array($char, $aEnd)) {
+            if (in_array($char, $aEnd, true)) {
                 if ($bIncludeEnd) {
                     $out .= $char;
                 } elseif (!$consumeEnd) {
@@ -386,7 +386,7 @@ public function consumeUntil($aEnd, $bIncludeEnd = false, $consumeEnd = false, a
             }
         }
 
-        if (in_array(self::EOF, $aEnd)) {
+        if (in_array(self::EOF, $aEnd, true)) {
             return $out;
         }
 
diff --git a/src/RuleSet/DeclarationBlock.php b/src/RuleSet/DeclarationBlock.php
index 44b1bb6a..73796fd0 100644
--- a/src/RuleSet/DeclarationBlock.php
+++ b/src/RuleSet/DeclarationBlock.php
@@ -60,14 +60,14 @@ public static function parse(ParserState $oParserState, $oList = null)
             do {
                 $aSelectorParts[] = $oParserState->consume(1)
                     . $oParserState->consumeUntil(['{', '}', '\'', '"'], false, false, $aComments);
-                if (in_array($oParserState->peek(), ['\'', '"']) && substr(end($aSelectorParts), -1) != "\\") {
+                if (in_array($oParserState->peek(), ['\'', '"'], true) && substr(end($aSelectorParts), -1) != "\\") {
                     if ($sStringWrapperChar === false) {
                         $sStringWrapperChar = $oParserState->peek();
                     } elseif ($sStringWrapperChar == $oParserState->peek()) {
                         $sStringWrapperChar = false;
                     }
                 }
-            } while (!in_array($oParserState->peek(), ['{', '}']) || $sStringWrapperChar !== false);
+            } while (!in_array($oParserState->peek(), ['{', '}'], true) || $sStringWrapperChar !== false);
             $oResult->setSelectors(implode('', $aSelectorParts), $oList);
             if ($oParserState->comes('{')) {
                 $oParserState->consume(1);
@@ -229,7 +229,7 @@ public function expandBorderShorthand(): void
                 } elseif ($mValue instanceof Color) {
                     $sNewRuleName = $sBorderRule . "-color";
                 } else {
-                    if (in_array($mValue, $aBorderSizes)) {
+                    if (in_array($mValue, $aBorderSizes, true)) {
                         $sNewRuleName = $sBorderRule . "-width";
                     } else {
                         $sNewRuleName = $sBorderRule . "-style";
@@ -338,20 +338,19 @@ public function expandFontShorthand(): void
             if (!$mValue instanceof Value) {
                 $mValue = mb_strtolower($mValue);
             }
-            if (in_array($mValue, ['normal', 'inherit'])) {
+            if (in_array($mValue, ['normal', 'inherit'], true)) {
                 foreach (['font-style', 'font-weight', 'font-variant'] as $sProperty) {
                     if (!isset($aFontProperties[$sProperty])) {
                         $aFontProperties[$sProperty] = $mValue;
                     }
                 }
-            } elseif (in_array($mValue, ['italic', 'oblique'])) {
+            } elseif (in_array($mValue, ['italic', 'oblique'], true)) {
                 $aFontProperties['font-style'] = $mValue;
             } elseif ($mValue == 'small-caps') {
                 $aFontProperties['font-variant'] = $mValue;
             } elseif (
-                in_array($mValue, ['bold', 'bolder', 'lighter'])
-                || ($mValue instanceof Size
-                    && in_array($mValue->getSize(), range(100, 900, 100)))
+                in_array($mValue, ['bold', 'bolder', 'lighter'], true)
+                || ($mValue instanceof Size && in_array($mValue->getSize(), range(100.0, 900.0, 100.0), true))
             ) {
                 $aFontProperties['font-weight'] = $mValue;
             } elseif ($mValue instanceof RuleValueList && $mValue->getListSeparator() == '/') {
@@ -425,12 +424,12 @@ public function expandBackgroundShorthand(): void
                 $aBgProperties['background-image'] = $mValue;
             } elseif ($mValue instanceof Color) {
                 $aBgProperties['background-color'] = $mValue;
-            } elseif (in_array($mValue, ['scroll', 'fixed'])) {
+            } elseif (in_array($mValue, ['scroll', 'fixed'], true)) {
                 $aBgProperties['background-attachment'] = $mValue;
-            } elseif (in_array($mValue, ['repeat', 'no-repeat', 'repeat-x', 'repeat-y'])) {
+            } elseif (in_array($mValue, ['repeat', 'no-repeat', 'repeat-x', 'repeat-y'], true)) {
                 $aBgProperties['background-repeat'] = $mValue;
             } elseif (
-                in_array($mValue, ['left', 'center', 'right', 'top', 'bottom'])
+                in_array($mValue, ['left', 'center', 'right', 'top', 'bottom'], true)
                 || $mValue instanceof Size
             ) {
                 if ($iNumBgPos == 0) {
@@ -516,9 +515,9 @@ public function expandListStyleShorthand(): void
             }
             if ($mValue instanceof Url) {
                 $aListProperties['list-style-image'] = $mValue;
-            } elseif (in_array($mValue, $aListStyleTypes)) {
+            } elseif (in_array($mValue, $aListStyleTypes, true)) {
                 $aListProperties['list-style-types'] = $mValue;
-            } elseif (in_array($mValue, $aListStylePositions)) {
+            } elseif (in_array($mValue, $aListStylePositions, true)) {
                 $aListProperties['list-style-position'] = $mValue;
             }
         }
diff --git a/src/Value/CalcFunction.php b/src/Value/CalcFunction.php
index 85f4085a..7487edf5 100644
--- a/src/Value/CalcFunction.php
+++ b/src/Value/CalcFunction.php
@@ -29,7 +29,7 @@ public static function parse(ParserState $oParserState, bool $bIgnoreCase = fals
         if ($oParserState->peek() != '(') {
             // Found ; or end of line before an opening bracket
             throw new UnexpectedTokenException('(', $oParserState->peek(), 'literal', $oParserState->currentLine());
-        } elseif (!in_array($sFunction, ['calc', '-moz-calc', '-webkit-calc'])) {
+        } elseif (!in_array($sFunction, ['calc', '-moz-calc', '-webkit-calc'], true)) {
             // Found invalid calc definition. Example calc (...
             throw new UnexpectedTokenException('calc', $sFunction, 'literal', $oParserState->currentLine());
         }
@@ -60,7 +60,7 @@ public static function parse(ParserState $oParserState, bool $bIgnoreCase = fals
                 $oCalcList->addListComponent($oVal);
                 $iLastComponentType = CalcFunction::T_OPERAND;
             } else {
-                if (in_array($oParserState->peek(), $aOperators)) {
+                if (in_array($oParserState->peek(), $aOperators, true)) {
                     if (($oParserState->comes('-') || $oParserState->comes('+'))) {
                         if (
                             $oParserState->peek(1, -1) != ' '