diff --git a/phpBB/language/en/posting.php b/phpBB/language/en/posting.php
index 426475e77a..570cf63f17 100644
--- a/phpBB/language/en/posting.php
+++ b/phpBB/language/en/posting.php
@@ -140,6 +140,7 @@ $lang = array_merge($lang, array(
 	'IMAGES_ARE_OFF'			=> '[img] is <em>OFF</em>',
 	'IMAGES_ARE_ON'				=> '[img] is <em>ON</em>',
 	'INVALID_FILENAME'			=> '%s is an invalid filename.',
+	'INVALID_FONT_SIZE'			=> 'The font size you supplied is invalid: %s',
 
 	'LOAD'						=> 'Load',
 	'LOAD_DRAFT'				=> 'Load draft',
diff --git a/phpBB/phpbb/textformatter/s9e/factory.php b/phpBB/phpbb/textformatter/s9e/factory.php
index 6191b9a315..dca1c78d40 100644
--- a/phpBB/phpbb/textformatter/s9e/factory.php
+++ b/phpBB/phpbb/textformatter/s9e/factory.php
@@ -110,7 +110,7 @@ class factory implements \phpbb\textformatter\cache_interface
 		'i'     => '<span style="font-style: italic"><xsl:apply-templates/></span>',
 		'u'     => '<span style="text-decoration: underline"><xsl:apply-templates/></span>',
 		'img'   => '<img src="{IMAGEURL}" class="postimage" alt="{L_IMAGE}"/>',
-		'size'  => '<span style="font-size: {FONTSIZE}%; line-height: normal"><xsl:apply-templates/></span>',
+		'size'	=> '<span><xsl:attribute name="style"><xsl:text>font-size: </xsl:text><xsl:value-of select="substring(@size, 1, 4)"/><xsl:text>%; line-height: normal</xsl:text></xsl:attribute><xsl:apply-templates/></span>',
 		'color' => '<span style="color: {COLOR}"><xsl:apply-templates/></span>',
 		'email' => '<a>
 			<xsl:attribute name="href">
diff --git a/phpBB/phpbb/textformatter/s9e/parser.php b/phpBB/phpbb/textformatter/s9e/parser.php
index 3698dca224..1bc56a8cb4 100644
--- a/phpBB/phpbb/textformatter/s9e/parser.php
+++ b/phpBB/phpbb/textformatter/s9e/parser.php
@@ -228,6 +228,10 @@ class parser implements \phpbb\textformatter\parser_interface
 			{
 				$errors[] = array($msg);
 			}
+			else if ($msg === 'INVALID_FONT_SIZE')
+			{
+				$errors[] = [$msg, $context['invalid_size']];
+			}
 		}
 
 		// Deduplicate error messages. array_unique() only works on strings so we have to serialize
@@ -335,6 +339,13 @@ class parser implements \phpbb\textformatter\parser_interface
 	*/
 	static public function filter_font_size($size, $max_size, Logger $logger)
 	{
+		if (!is_numeric($size))
+		{
+			$logger->err('INVALID_FONT_SIZE', ['invalid_size' => htmlspecialchars($size)]);
+
+			return false;
+		}
+
 		if ($max_size && $size > $max_size)
 		{
 			$logger->err('MAX_FONT_SIZE_EXCEEDED', array('max_size' => $max_size));
diff --git a/phpBB/styles/prosilver/template/bbcode.html b/phpBB/styles/prosilver/template/bbcode.html
index 940c0ace29..f4ec94dbfe 100644
--- a/phpBB/styles/prosilver/template/bbcode.html
+++ b/phpBB/styles/prosilver/template/bbcode.html
@@ -64,7 +64,7 @@
 
 <!-- BEGIN color --><span style="color: {COLOR}">{TEXT}</span><!-- END color -->
 
-<!-- BEGIN size --><span style="font-size: {SIZE}%; line-height: 116%;">{TEXT}</span><!-- END size -->
+<!-- BEGIN size --><span><xsl:attribute name="style"><xsl:text>font-size: </xsl:text><xsl:value-of select="substring(@size, 1, 4)"/><xsl:text>%; line-height: normal</xsl:text></xsl:attribute><xsl:apply-templates/></span><!-- END size -->
 
 <!-- BEGIN img --><img src="{URL}" class="postimage" alt="{L_IMAGE}" /><!-- END img -->
 
diff --git a/tests/text_formatter/s9e/default_formatting_test.php b/tests/text_formatter/s9e/default_formatting_test.php
index a35c9138a5..1aa4f0bc3a 100644
--- a/tests/text_formatter/s9e/default_formatting_test.php
+++ b/tests/text_formatter/s9e/default_formatting_test.php
@@ -70,7 +70,7 @@ class phpbb_textformatter_s9e_default_formatting_test extends phpbb_test_case
 			),
 			array(
 				'[size=75]smaller[/size]',
-				'<span style="font-size:75%;line-height:normal">smaller</span>'
+				'<span style="font-size: 75%; line-height: normal">smaller</span>'
 			),
 			array(
 				'[quote]quoted[/quote]',
diff --git a/tests/text_processing/tickets_data/PHPBB3-13921.html b/tests/text_processing/tickets_data/PHPBB3-13921.html
index 690668ef28..6a9dc7f504 100644
--- a/tests/text_processing/tickets_data/PHPBB3-13921.html
+++ b/tests/text_processing/tickets_data/PHPBB3-13921.html
@@ -1 +1 @@
-<span style="font-size:200%;line-height:normal"></span><div style="text-align:center"><span style="font-size:200%;line-height:normal">xxx</span></div>
\ No newline at end of file
+<span style="font-size: 200%; line-height: normal"></span><div style="text-align:center"><span style="font-size: 200%; line-height: normal">xxx</span></div>
\ No newline at end of file