diff --git a/core/lib/Drupal/Core/Asset/CssOptimizer.php b/core/lib/Drupal/Core/Asset/CssOptimizer.php
index 4ad7ba1..6b07a52 100644
--- a/core/lib/Drupal/Core/Asset/CssOptimizer.php
+++ b/core/lib/Drupal/Core/Asset/CssOptimizer.php
@@ -60,8 +60,9 @@ protected function processFile($css_asset) {
     // Store base path.
     $this->rewriteFileURIBasePath = $css_base_path . '/';
 
-    // Anchor all paths in the CSS with its base URL, ignoring external and absolute paths.
-    return preg_replace_callback('/url\(\s*[\'"]?(?![a-z]+:|\/+)([^\'")]+)[\'"]?\s*\)/i', [$this, 'rewriteFileURI'], $contents);
+    // Anchor all paths in the CSS with its base URL, ignoring external,
+    // absolute, and SVG (# or %23) paths.
+    return preg_replace_callback('/url\(\s*[\'"]?(?![a-z]+:|\/+|\#|\%23+)([^\'")]+)[\'"]?\s*\)/i', [$this, 'rewriteFileURI'], $contents);
   }
 
   /**
@@ -166,8 +167,9 @@ protected function loadNestedFile($matches) {
     $directory = $directory == '.' ? '' : $directory . '/';
 
     // Alter all internal url() paths. Leave external paths alone. We don't need
-    // to normalize absolute paths here because that will be done later.
-    return preg_replace('/url\(\s*([\'"]?)(?![a-z]+:|\/+)([^\'")]+)([\'"]?)\s*\)/i', 'url(\1' . $directory . '\2\3)', $file);
+    // to normalize absolute paths here because that will be done later. Also
+    // ignore SVG paths (# or %23).
+    return preg_replace('/url\(\s*([\'"]?)(?![a-z]+:|\/+|\#|\%23+)([^\'")]+)([\'"]?)\s*\)/i', 'url(\1' . $directory . '\2\3)', $file);
   }
 
   /**
diff --git a/core/modules/color/color.module b/core/modules/color/color.module
index 20d099a..2da7450 100644
--- a/core/modules/color/color.module
+++ b/core/modules/color/color.module
@@ -488,8 +488,9 @@ function color_scheme_form_submit($form, FormStateInterface $form_state) {
       // off the name of the file at the end of the path.
       $css_optimizer->rewriteFileURIBasePath = base_path() . dirname($paths['source'] . $file) . '/';
 
-      // Prefix all paths within this CSS file, ignoring absolute paths.
-      $style = preg_replace_callback('/url\([\'"]?(?![a-z]+:|\/+)([^\'")]+)[\'"]?\)/i', [$css_optimizer, 'rewriteFileURI'], $style);
+      // Prefix all paths within this CSS file, ignoring external, absolute, and
+      // SVG paths (# or %23).
+      $style = preg_replace_callback('/url\([\'"]?(?![a-z]+:|\/+|\#|\%23+)([^\'")]+)[\'"]?\)/i', [$css_optimizer, 'rewriteFileURI'], $style);
 
       // Rewrite stylesheet with new colors.
       $style = _color_rewrite_stylesheet($theme, $info, $paths, $palette, $style);
diff --git a/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_input_with_import.css.optimized.css b/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_input_with_import.css.optimized.css
index 16bd93b..e95e104 100644
--- a/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_input_with_import.css.optimized.css
+++ b/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_input_with_import.css.optimized.css
@@ -1,4 +1,5 @@
-ul,select{font:1em/160% Verdana,sans-serif;color:#494949;}.ui-icon{background-image:url(images/icon.png);}.data .double-quote{background-image:url("data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7");}.data .single-quote{background-image:url('data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDACAWGBwYFCAcGhwkIiAmMFA0MCwsMGJGSjpQdGZ6eHJmcG6AkLicgIiuim5woNqirr7EztDOfJri8uDI8LjKzsb/2wBDASIkJDAqMF40NF7GhHCExsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsb/wAARCAABAAEDASIAAhEBAxEB/8QAFQABAQAAAAAAAAAAAAAAAAAAAAb/xAAUEAEAAAAAAAAAAAAAAAAAAAAA/8QAFAEBAAAAAAAAAAAAAAAAAAAAAP/EABQRAQAAAAAAAAAAAAAAAAAAAAD/2gAMAwEAAhEDEQA/AKAAH//Z');}.data .no-quote{background-image:url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACEAAAAEAQAAAAAo/mtHAAAAIElEQVQIHWMRnWHwcRNLN8NZ7QYWwT8PlBlYsgqVBRsAankIMw5MtnoAAAAASUVORK5CYII=);}
+ul,select{font:1em/160% Verdana,sans-serif;color:#494949;}.ui-icon{background-image:url(images/icon.png);}.data .double-quote{background-image:url("data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7");}.data .single-quote{background-image:url('data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDACAWGBwYFCAcGhwkIiAmMFA0MCwsMGJGSjpQdGZ6eHJmcG6AkLicgIiuim5woNqirr7EztDOfJri8uDI8LjKzsb/2wBDASIkJDAqMF40NF7GhHCExsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsb/wAARCAABAAEDASIAAhEBAxEB/8QAFQABAQAAAAAAAAAAAAAAAAAAAAb/xAAUEAEAAAAAAAAAAAAAAAAAAAAA/8QAFAEBAAAAAAAAAAAAAAAAAAAAAP/EABQRAQAAAAAAAAAAAAAAAAAAAAD/2gAMAwEAAhEDEQA/AKAAH//Z');}.data .no-quote{background-image:url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACEAAAAEAQAAAAAo/mtHAAAAIElEQVQIHWMRnWHwcRNLN8NZ7QYWwT8PlBlYsgqVBRsAankIMw5MtnoAAAAASUVORK5CYII=);}.url .no-quote-1{background:url(#SVGID_1_);}.url .no-quote-2{background:url(#a);}.url .single-quote{background:url('#a');}.url .double-quote{background:url("#a");}.url .double-quote-with-spaces{background:url("#a");}.url .no-quotes-with-spaces{background:url(#a);}.url .inline-svg{background-image:url('data:image/svg+xml;charset=US-ASCII,%3C%3Fxml%20version%3D%221.0%22%20encoding%3D%22utf-8%22%3F%3E%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%222000%22%20height%3D%2221%22%20viewBox%3D%22-423.5%200%202000%2021%22%20enable-background%3D%22new%20-423.5%200%202000%2021%22%3E%3ClinearGradient%20id%3D%22SVGID_1_%22%20gradientUnits%3D%22userSpaceOnUse%22%20x1%3D%221393.788%22%20y1%3D%22404.834%22%20x2%3D%221395.121%22%20y2%3D%22404.834%22%20gradientTransform%3D%22matrix(1500%200%200%20-538%20-2091105%20217811)%22%3E%3Cstop%20offset%3D%220%22%20stop-color%3D%22%23fff%22%20stop-opacity%3D%220%22%2F%3E%3Cstop%20offset%3D%220%22%20stop-color%3D%22%23fff%22%2F%3E%3Cstop%20offset%3D%22.55%22%20stop-color%3D%22%23fff%22%2F%3E%3Cstop%20offset%3D%22.8%22%20stop-color%3D%22%23fff%22%20stop-opacity%3D%220%22%2F%3E%3Cstop%20offset%3D%221%22%20stop-color%3D%22%23fff%22%20stop-opacity%3D%220%22%2F%3E%3C%2FlinearGradient%3E%3Cpath%20fill%3D%22url(%23SVGID_1_)%22%20d%3D%22M-423.
+5%200h2000v21h-2000V0z%22%2F%3E%3C%2Fsvg%3E');}
 p,select{font:1em/160% Verdana,sans-serif;color:#494949;}
 @import url("http://example.com/style.css");@import url("//example.com/style.css");body{margin:0;padding:0;background:#edf5fa;font:76%/170% Verdana,sans-serif;color:#494949;}.this .is .a .test{font:1em/100% Verdana,sans-serif;color:#494949;}.this
 .is
diff --git a/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_subfolder/css_input_with_import.css.optimized.css b/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_subfolder/css_input_with_import.css.optimized.css
index 816039d..7b853a4 100644
--- a/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_subfolder/css_input_with_import.css.optimized.css
+++ b/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_subfolder/css_input_with_import.css.optimized.css
@@ -1,4 +1,5 @@
-ul,select{font:1em/160% Verdana,sans-serif;color:#494949;}.ui-icon{background-image:url(../images/icon.png);}.data .double-quote{background-image:url("data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7");}.data .single-quote{background-image:url('data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDACAWGBwYFCAcGhwkIiAmMFA0MCwsMGJGSjpQdGZ6eHJmcG6AkLicgIiuim5woNqirr7EztDOfJri8uDI8LjKzsb/2wBDASIkJDAqMF40NF7GhHCExsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsb/wAARCAABAAEDASIAAhEBAxEB/8QAFQABAQAAAAAAAAAAAAAAAAAAAAb/xAAUEAEAAAAAAAAAAAAAAAAAAAAA/8QAFAEBAAAAAAAAAAAAAAAAAAAAAP/EABQRAQAAAAAAAAAAAAAAAAAAAAD/2gAMAwEAAhEDEQA/AKAAH//Z');}.data .no-quote{background-image:url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACEAAAAEAQAAAAAo/mtHAAAAIElEQVQIHWMRnWHwcRNLN8NZ7QYWwT8PlBlYsgqVBRsAankIMw5MtnoAAAAASUVORK5CYII=);}
+ul,select{font:1em/160% Verdana,sans-serif;color:#494949;}.ui-icon{background-image:url(../images/icon.png);}.data .double-quote{background-image:url("data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7");}.data .single-quote{background-image:url('data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDACAWGBwYFCAcGhwkIiAmMFA0MCwsMGJGSjpQdGZ6eHJmcG6AkLicgIiuim5woNqirr7EztDOfJri8uDI8LjKzsb/2wBDASIkJDAqMF40NF7GhHCExsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsbGxsb/wAARCAABAAEDASIAAhEBAxEB/8QAFQABAQAAAAAAAAAAAAAAAAAAAAb/xAAUEAEAAAAAAAAAAAAAAAAAAAAA/8QAFAEBAAAAAAAAAAAAAAAAAAAAAP/EABQRAQAAAAAAAAAAAAAAAAAAAAD/2gAMAwEAAhEDEQA/AKAAH//Z');}.data .no-quote{background-image:url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACEAAAAEAQAAAAAo/mtHAAAAIElEQVQIHWMRnWHwcRNLN8NZ7QYWwT8PlBlYsgqVBRsAankIMw5MtnoAAAAASUVORK5CYII=);}.url .no-quote-1{background:url(#SVGID_1_);}.url .no-quote-2{background:url(#a);}.url .single-quote{background:url('#a');}.url .double-quote{background:url("#a");}.url .double-quote-with-spaces{background:url("#a");}.url .no-quotes-with-spaces{background:url(#a);}.url .inline-svg{background-image:url('data:image/svg+xml;charset=US-ASCII,%3C%3Fxml%20version%3D%221.0%22%20encoding%3D%22utf-8%22%3F%3E%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%222000%22%20height%3D%2221%22%20viewBox%3D%22-423.5%200%202000%2021%22%20enable-background%3D%22new%20-423.5%200%202000%2021%22%3E%3ClinearGradient%20id%3D%22SVGID_1_%22%20gradientUnits%3D%22userSpaceOnUse%22%20x1%3D%221393.788%22%20y1%3D%22404.834%22%20x2%3D%221395.121%22%20y2%3D%22404.834%22%20gradientTransform%3D%22matrix(1500%200%200%20-538%20-2091105%20217811)%22%3E%3Cstop%20offset%3D%220%22%20stop-color%3D%22%23fff%22%20stop-opacity%3D%220%22%2F%3E%3Cstop%20offset%3D%220%22%20stop-color%3D%22%23fff%22%2F%3E%3Cstop%20offset%3D%22.55%22%20stop-color%3D%22%23fff%22%2F%3E%3Cstop%20offset%3D%22.8%22%20stop-color%3D%22%23fff%22%20stop-opacity%3D%220%22%2F%3E%3Cstop%20offset%3D%221%22%20stop-color%3D%22%23fff%22%20stop-opacity%3D%220%22%2F%3E%3C%2FlinearGradient%3E%3Cpath%20fill%3D%22url(%23SVGID_1_)%22%20d%3D%22M-423.
+5%200h2000v21h-2000V0z%22%2F%3E%3C%2Fsvg%3E');}
 p,select{font:1em/160% Verdana,sans-serif;color:#494949;}
 body{margin:0;padding:0;background:#edf5fa;font:76%/170% Verdana,sans-serif;color:#494949;}.this .is .a .test{font:1em/100% Verdana,sans-serif;color:#494949;}.this
 .is
diff --git a/core/tests/Drupal/Tests/Core/Asset/css_test_files/import1.css b/core/tests/Drupal/Tests/Core/Asset/css_test_files/import1.css
index e53d6d5..ca39033 100644
--- a/core/tests/Drupal/Tests/Core/Asset/css_test_files/import1.css
+++ b/core/tests/Drupal/Tests/Core/Asset/css_test_files/import1.css
@@ -18,3 +18,33 @@ ul, select {
 .data .no-quote {
   background-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACEAAAAEAQAAAAAo/mtHAAAAIElEQVQIHWMRnWHwcRNLN8NZ7QYWwT8PlBlYsgqVBRsAankIMw5MtnoAAAAASUVORK5CYII=);
 }
+
+/* Test urls with # or %23 character - https://www.drupal.org/node/2362643 */
+.url .no-quote-1 {
+  background: url(#SVGID_1_);
+}
+
+.url .no-quote-2 {
+  background: url(#a);
+}
+
+.url .single-quote {
+  background: url('#a');
+}
+
+.url .double-quote {
+  background: url("#a");
+}
+
+.url .double-quote-with-spaces {
+  background: url( "#a" );
+}
+
+.url .no-quotes-with-spaces {
+  background: url( #a );
+}
+
+.url .inline-svg {
+  background-image: url('data:image/svg+xml;charset=US-ASCII,%3C%3Fxml%20version%3D%221.0%22%20encoding%3D%22utf-8%22%3F%3E%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%222000%22%20height%3D%2221%22%20viewBox%3D%22-423.5%200%202000%2021%22%20enable-background%3D%22new%20-423.5%200%202000%2021%22%3E%3ClinearGradient%20id%3D%22SVGID_1_%22%20gradientUnits%3D%22userSpaceOnUse%22%20x1%3D%221393.788%22%20y1%3D%22404.834%22%20x2%3D%221395.121%22%20y2%3D%22404.834%22%20gradientTransform%3D%22matrix(1500%200%200%20-538%20-2091105%20217811)%22%3E%3Cstop%20offset%3D%220%22%20stop-color%3D%22%23fff%22%20stop-opacity%3D%220%22%2F%3E%3Cstop%20offset%3D%220%22%20stop-color%3D%22%23fff%22%2F%3E%3Cstop%20offset%3D%22.55%22%20stop-color%3D%22%23fff%22%2F%3E%3Cstop%20offset%3D%22.8%22%20stop-color%3D%22%23fff%22%20stop-opacity%3D%220%22%2F%3E%3Cstop%20offset%3D%221%22%20stop-color%3D%22%23fff%22%20stop-opacity%3D%220%22%2F%3E%3C%2FlinearGradient%3E%3Cpath%20fill%3D%22url(%23SVGID_1_)%22%20d%3D%22M-423.
+5%200h2000v21h-2000V0z%22%2F%3E%3C%2Fsvg%3E');
+}