diff --git a/src/Spout/Common/Entity/Cell.php b/src/Spout/Common/Entity/Cell.php
index c1de389..174831c 100644
--- a/src/Spout/Common/Entity/Cell.php
+++ b/src/Spout/Common/Entity/Cell.php
@@ -65,7 +65,7 @@ class Cell
     protected $style;
 
     /**
-     * @param $value mixed
+     * @param mixed|null $value
      * @param Style|null $style
      */
     public function __construct($value, Style $style = null)
diff --git a/src/Spout/Reader/XLSX/Helper/CellValueFormatter.php b/src/Spout/Reader/XLSX/Helper/CellValueFormatter.php
index 169c395..e95f186 100644
--- a/src/Spout/Reader/XLSX/Helper/CellValueFormatter.php
+++ b/src/Spout/Reader/XLSX/Helper/CellValueFormatter.php
@@ -31,14 +31,6 @@ class CellValueFormatter
 
     /** Constants used for date formatting */
     const NUM_SECONDS_IN_ONE_DAY = 86400;
-    const NUM_SECONDS_IN_ONE_HOUR = 3600;
-    const NUM_SECONDS_IN_ONE_MINUTE = 60;
-
-    /**
-     * February 29th, 1900 is NOT a leap year but Excel thinks it is...
-     * @see https://en.wikipedia.org/wiki/Year_1900_problem#Microsoft_Excel
-     */
-    const ERRONEOUS_EXCEL_LEAP_YEAR_DAY = 60;
 
     /** @var SharedStringsManager Manages shared strings */
     protected $sharedStringsManager;
diff --git a/src/Spout/Reader/XLSX/Manager/SharedStringsManager.php b/src/Spout/Reader/XLSX/Manager/SharedStringsManager.php
index 8850a69..81b4ba2 100644
--- a/src/Spout/Reader/XLSX/Manager/SharedStringsManager.php
+++ b/src/Spout/Reader/XLSX/Manager/SharedStringsManager.php
@@ -16,9 +16,6 @@ use Box\Spout\Reader\XLSX\Manager\SharedStringsCaching\CachingStrategyInterface;
  */
 class SharedStringsManager
 {
-    /** Main namespace for the sharedStrings.xml file */
-    const MAIN_NAMESPACE_FOR_SHARED_STRINGS_XML = 'http://schemas.openxmlformats.org/spreadsheetml/2006/main';
-
     /** Definition of XML nodes names used to parse data */
     const XML_NODE_SST = 'sst';
     const XML_NODE_SI = 'si';
diff --git a/src/Spout/Reader/XLSX/Manager/WorkbookRelationshipsManager.php b/src/Spout/Reader/XLSX/Manager/WorkbookRelationshipsManager.php
index a153d78..0a9f6f5 100644
--- a/src/Spout/Reader/XLSX/Manager/WorkbookRelationshipsManager.php
+++ b/src/Spout/Reader/XLSX/Manager/WorkbookRelationshipsManager.php
@@ -17,10 +17,11 @@ class WorkbookRelationshipsManager
     /** Path of workbook relationships XML file inside the XLSX file */
     const WORKBOOK_RELS_XML_FILE_PATH = 'xl/_rels/workbook.xml.rels';
 
-    /** Relationships types */
+    /** Relationships types - For Transitional and Strict OOXML */
     const RELATIONSHIP_TYPE_SHARED_STRINGS = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/sharedStrings';
     const RELATIONSHIP_TYPE_STYLES = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/styles';
-    const RELATIONSHIP_TYPE_WORKSHEET = 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet';
+    const RELATIONSHIP_TYPE_SHARED_STRINGS_STRICT = 'http://purl.oclc.org/ooxml/officeDocument/relationships/sharedStrings';
+    const RELATIONSHIP_TYPE_STYLES_STRICT = 'http://purl.oclc.org/ooxml/officeDocument/relationships/styles';
 
     /** Nodes and attributes used to find relevant information in the workbook relationships XML file */
     const XML_NODE_RELATIONSHIP = 'Relationship';
@@ -52,7 +53,8 @@ class WorkbookRelationshipsManager
     public function getSharedStringsXMLFilePath()
     {
         $workbookRelationships = $this->getWorkbookRelationships();
-        $sharedStringsXMLFilePath = $workbookRelationships[self::RELATIONSHIP_TYPE_SHARED_STRINGS];
+        $sharedStringsXMLFilePath = $workbookRelationships[self::RELATIONSHIP_TYPE_SHARED_STRINGS]
+            ?? $workbookRelationships[self::RELATIONSHIP_TYPE_SHARED_STRINGS_STRICT];
 
         // the file path can be relative (e.g. "styles.xml") or absolute (e.g. "/xl/styles.xml")
         $doesContainBasePath = (\strpos($sharedStringsXMLFilePath, self::BASE_PATH) !== false);
@@ -71,7 +73,8 @@ class WorkbookRelationshipsManager
     {
         $workbookRelationships = $this->getWorkbookRelationships();
 
-        return isset($workbookRelationships[self::RELATIONSHIP_TYPE_SHARED_STRINGS]);
+        return isset($workbookRelationships[self::RELATIONSHIP_TYPE_SHARED_STRINGS])
+            || isset($workbookRelationships[self::RELATIONSHIP_TYPE_SHARED_STRINGS_STRICT]);
     }
 
     /**
@@ -81,7 +84,8 @@ class WorkbookRelationshipsManager
     {
         $workbookRelationships = $this->getWorkbookRelationships();
 
-        return isset($workbookRelationships[self::RELATIONSHIP_TYPE_STYLES]);
+        return isset($workbookRelationships[self::RELATIONSHIP_TYPE_STYLES])
+            || isset($workbookRelationships[self::RELATIONSHIP_TYPE_STYLES_STRICT]);
     }
 
     /**
@@ -90,7 +94,8 @@ class WorkbookRelationshipsManager
     public function getStylesXMLFilePath()
     {
         $workbookRelationships = $this->getWorkbookRelationships();
-        $stylesXMLFilePath = $workbookRelationships[self::RELATIONSHIP_TYPE_STYLES];
+        $stylesXMLFilePath = $workbookRelationships[self::RELATIONSHIP_TYPE_STYLES]
+            ?? $workbookRelationships[self::RELATIONSHIP_TYPE_STYLES_STRICT];
 
         // the file path can be relative (e.g. "styles.xml") or absolute (e.g. "/xl/styles.xml")
         $doesContainBasePath = (\strpos($stylesXMLFilePath, self::BASE_PATH) !== false);
diff --git a/tests/Spout/Reader/XLSX/ReaderTest.php b/tests/Spout/Reader/XLSX/ReaderTest.php
index 7d30f97..e9888d8 100644
--- a/tests/Spout/Reader/XLSX/ReaderTest.php
+++ b/tests/Spout/Reader/XLSX/ReaderTest.php
@@ -703,6 +703,18 @@ class ReaderTest extends TestCase
         $this->assertEquals($expectedRows, $allRows, 'Cell values should not be trimmed');
     }
 
+    /**
+     * https://github.com/box/spout/issues/726
+     * @return void
+     */
+    public function testReaderShouldSupportStrictOOXML()
+    {
+        $allRows = $this->getAllRowsForFile('sheet_with_strict_ooxml.xlsx');
+
+        $this->assertEquals('UNIQUE_ACCOUNT_IDENTIFIER', $allRows[0][0]);
+        $this->assertEquals('A2Z34NJA7N2ESJ', $allRows[1][0]);
+    }
+
     /**
      * @param string $fileName
      * @param bool $shouldFormatDates
diff --git a/tests/resources/xlsx/sheet_with_strict_ooxml.xlsx b/tests/resources/xlsx/sheet_with_strict_ooxml.xlsx
new file mode 100644
index 0000000..0fba9f9
Binary files /dev/null and b/tests/resources/xlsx/sheet_with_strict_ooxml.xlsx differ