diff --git a/.jscsrc b/.jscsrc
index 6425f203..9a7bbb01 100644
--- a/.jscsrc
+++ b/.jscsrc
@@ -2,6 +2,6 @@
     "preset": "wordpress",
     "fileExtensions": [ ".js" ],
     "excludeFiles": [
-    	"js/skip-link-focus-fix.js"
+    	"js/**.min.js"
     ]
 }
\ No newline at end of file
diff --git a/.jshintignore b/.jshintignore
index 92af1049..54162f6f 100644
--- a/.jshintignore
+++ b/.jshintignore
@@ -1,2 +1 @@
-js/skip-link-focus-fix.js
-**.min.js
\ No newline at end of file
+js/**.min.js
\ No newline at end of file
diff --git a/.jshintrc b/.jshintrc
deleted file mode 100644
index 012652e9..00000000
--- a/.jshintrc
+++ /dev/null
@@ -1,26 +0,0 @@
-{
-	"boss": true,
-	"curly": true,
-	"eqeqeq": true,
-	"eqnull": true,
-	"es3": true,
-	"expr": true,
-	"immed": true,
-	"noarg": true,
-	"nonbsp": true,
-	"onevar": true,
-	"quotmark": "single",
-	"trailing": true,
-	"undef": true,
-	"unused": true,
-
-	"browser": true,
-
-	"globals": {
-		"_": false,
-		"Backbone": false,
-		"jQuery": false,
-		"JSON": false,
-		"wp": false
-	}
-}
diff --git a/.travis.yml b/.travis.yml
index b0e38c28..4499a99d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -22,15 +22,9 @@ matrix:
         # Current $required_php_version for WordPress: 5.2.4
         # aliased to 5.2.17
         - php: '5.2'
-        # aliased to 5.3.29
-        - php: '5.3'
-        # aliased to a recent 5.4.x version
-        - php: '5.4'
-        # aliased to a recent 5.5.x version
-        - php: '5.5'
-          env: SNIFF=1
         # aliased to a recent 5.6.x version
         - php: '5.6'
+          env: SNIFF=1
         # aliased to a recent 7.x version
         - php: '7.0'
         # aliased to a recent hhvm version
@@ -49,22 +43,26 @@ matrix:
 # Failures in this section will result in build status 'errored'.
 before_script:
     - export PHPCS_DIR=/tmp/phpcs
-    - export WPCS_DIR=/tmp/wpcs
+    - export SNIFFS_DIR=/tmp/sniffs
     # Install CodeSniffer for WordPress Coding Standards checks.
     - if [[ "$SNIFF" == "1" ]]; then git clone -b master --depth 1 https://github.com/squizlabs/PHP_CodeSniffer.git $PHPCS_DIR; fi
     # Install WordPress Coding Standards.
-    - if [[ "$SNIFF" == "1" ]]; then git clone -b master --depth 1 https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards.git $WPCS_DIR; fi
-    # Set install path for WordPress Coding Standards.
+    - if [[ "$SNIFF" == "1" ]]; then git clone -b master --depth 1 https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards.git $SNIFFS_DIR; fi
+    # Install PHP Compatibility sniffs.
+    - if [[ "$SNIFF" == "1" ]]; then git clone -b master --depth 1 https://github.com/wimg/PHPCompatibility.git $SNIFFS_DIR/PHPCompatibility; fi
+    # Set install path for PHPCS sniffs.
     # @link https://github.com/squizlabs/PHP_CodeSniffer/blob/4237c2fc98cc838730b76ee9cee316f99286a2a7/CodeSniffer.php#L1941
-    - if [[ "$SNIFF" == "1" ]]; then $PHPCS_DIR/scripts/phpcs --config-set installed_paths $WPCS_DIR; fi
+    - if [[ "$SNIFF" == "1" ]]; then $PHPCS_DIR/scripts/phpcs --config-set installed_paths $SNIFFS_DIR; fi
     # After CodeSniffer install you should refresh your path.
     - if [[ "$SNIFF" == "1" ]]; then phpenv rehash; fi
-    # Install JSCS: JavaScript Code Style checker
+    # Install JSCS: JavaScript Code Style checker.
     # @link http://jscs.info/
     - if [[ "$SNIFF" == "1" ]]; then npm install -g jscs; fi
-    # Install JSHint, a JavaScript Code Quality Tool
+    # Install JSHint, a JavaScript Code Quality Tool.
     # @link http://jshint.com/docs/
     - if [[ "$SNIFF" == "1" ]]; then npm install -g jshint; fi
+    # Pull in the WP Core jshint rules.
+    - wget https://develop.svn.wordpress.org/trunk/.jshintrc
 
 # Run test script commands.
 # Default is specific to project language.
diff --git a/codesniffer.ruleset.xml b/codesniffer.ruleset.xml
index b313a022..f16b3114 100644
--- a/codesniffer.ruleset.xml
+++ b/codesniffer.ruleset.xml
@@ -12,4 +12,7 @@
 		<exclude name="Generic.WhiteSpace.ScopeIndent.Incorrect" />
 		<exclude name="PEAR.Functions.FunctionCallSignature.Indent" />
 	</rule>
-</ruleset>
\ No newline at end of file
+
+	<!-- Include sniffs for PHP cross-version compatibility. -->
+	<rule ref="PHPCompatibility"/>
+</ruleset>
diff --git a/js/skip-link-focus-fix.js b/js/skip-link-focus-fix.js
index 44df0c4b..a39cefbf 100644
--- a/js/skip-link-focus-fix.js
+++ b/js/skip-link-focus-fix.js
@@ -1,16 +1,16 @@
 /**
- * skip-link-focus-fix.js
+ * File skip-link-focus-fix.js.
  *
  * Helps with accessibility for keyboard only users.
  *
  * Learn more: https://git.io/vWdr2
  */
 ( function() {
-	var is_webkit = navigator.userAgent.toLowerCase().indexOf( 'webkit' ) > -1,
-	    is_opera  = navigator.userAgent.toLowerCase().indexOf( 'opera' )  > -1,
-	    is_ie     = navigator.userAgent.toLowerCase().indexOf( 'msie' )   > -1;
+	var isWebkit = navigator.userAgent.toLowerCase().indexOf( 'webkit' ) > -1,
+	    isOpera  = navigator.userAgent.toLowerCase().indexOf( 'opera' )  > -1,
+	    isIe     = navigator.userAgent.toLowerCase().indexOf( 'msie' )   > -1;
 
-	if ( ( is_webkit || is_opera || is_ie ) && document.getElementById && window.addEventListener ) {
+	if ( ( isWebkit || isOpera || isIe ) && document.getElementById && window.addEventListener ) {
 		window.addEventListener( 'hashchange', function() {
 			var id = location.hash.substring( 1 ),
 				element;