Warn for inline style mismatches

I use the technique of generating a style string and comparing that against the
attribute.

scripts/fiber/tests-passing.txt

@@ -1531,6 +1531,13 @@ src/renderers/dom/shared/__tests__/ReactDOMServerIntegration-test.js
 * should error reconnecting missing attributes
 * should error reconnecting added attributes
 * should error reconnecting different attribute values
+* should error reconnecting missing style attribute
+* should error reconnecting added style attribute
+* should error reconnecting empty style attribute
+* should error reconnecting added style values
+* should error reconnecting different style values
+* should reconnect number and string versions of a number
+* should error reconnecting reordered style values
 * should error reconnecting different text
 * should reconnect a div with a number and string version of number
 * should error reconnecting different numbers

src/renderers/dom/fiber/ReactDOMFiberComponent.js

@@ -915,7 +915,13 @@ var ReactDOMFiberComponent = {
         } else if (propKey === STYLE) {
           // $FlowFixMe - Should be inferred as not undefined.
           extraAttributeNames.delete(propKey);
-          // TOOD: Validate style sheets.
+          const expectedStyle = CSSPropertyOperations.createDangerousStringForStyles(
+            nextProp,
+          );
+          serverValue = domElement.getAttribute('style');
+          if (expectedStyle !== serverValue) {
+            warnForPropDifference(propKey, serverValue, expectedStyle);
+          }
         } else if (
           isCustomComponentTag ||
           DOMProperty.isCustomAttribute(propKey)

src/renderers/dom/shared/CSSPropertyOperations.js

@@ -207,7 +207,6 @@ var CSSPropertyOperations = {
         serialized += dangerousStyleValue(
           styleName,
           styleValue,
-          component,
           isCustomProperty,
         );
 
@@ -217,6 +216,40 @@ var CSSPropertyOperations = {
     return serialized || null;
   },
 
+  /**
+   * This creates a string that is expected to be equivalent to the style
+   * attribute generated by server-side rendering. It by-passes warnings and
+   * security checks so it's not safe to use this value for anything other than
+   * comparison. It is only used in DEV for SSR validation. This is duplicated
+   * from createMarkupForStyles because createMarkupForStyles is expected to
+   * move out of the client-side renderer and it would be nice to make a clean
+   * break.
+   */
+  createDangerousStringForStyles: function(styles) {
+    if (__DEV__) {
+      var serialized = '';
+      var delimiter = '';
+      for (var styleName in styles) {
+        if (!styles.hasOwnProperty(styleName)) {
+          continue;
+        }
+        var styleValue = styles[styleName];
+        if (styleValue != null) {
+          var isCustomProperty = styleName.indexOf('--') === 0;
+          serialized += delimiter + hyphenateStyleName(styleName) + ':';
+          serialized += dangerousStyleValue(
+            styleName,
+            styleValue,
+            isCustomProperty,
+          );
+
+          delimiter = ';';
+        }
+      }
+      return serialized || null;
+    }
+  },
+
   /**
    * Sets the value for multiple styles on a node.  If a value is specified as
    * '' (empty string), the corresponding style property will be unset.
@@ -240,7 +273,6 @@ var CSSPropertyOperations = {
       var styleValue = dangerousStyleValue(
         styleName,
         styles[styleName],
-        component,
         isCustomProperty,
       );
       if (styleName === 'float') {

src/renderers/dom/shared/__tests__/ReactDOMServerIntegration-test.js

@@ -2075,6 +2075,44 @@ describe('ReactDOMServerIntegration', () => {
         expectMarkupMismatch(<div id="foo" />, <div id="bar" />));
     });
 
+    describe('inline styles', function() {
+      it('should error reconnecting missing style attribute', () =>
+        expectMarkupMismatch(<div style={{width: '1px'}} />, <div />));
+
+      it('should error reconnecting added style attribute', () =>
+        expectMarkupMismatch(<div />, <div style={{width: '1px'}} />));
+
+      it('should error reconnecting empty style attribute', () =>
+        expectMarkupMismatch(
+          <div style={{width: '1px'}} />,
+          <div style={{}} />,
+        ));
+
+      it('should error reconnecting added style values', () =>
+        expectMarkupMismatch(
+          <div style={{}} />,
+          <div style={{width: '1px'}} />,
+        ));
+
+      it('should error reconnecting different style values', () =>
+        expectMarkupMismatch(
+          <div style={{width: '1px'}} />,
+          <div style={{width: '2px'}} />,
+        ));
+
+      it('should reconnect number and string versions of a number', () =>
+        expectMarkupMatch(
+          <div style={{width: '1px', height: 2}} />,
+          <div style={{width: 1, height: '2px'}} />,
+        ));
+
+      it('should error reconnecting reordered style values', () =>
+        expectMarkupMismatch(
+          <div style={{width: '1px', fontSize: '2px'}} />,
+          <div style={{fontSize: '2px', width: '1px'}} />,
+        ));
+    });
+
     describe('text nodes', function() {
       it('should error reconnecting different text', () =>
         expectMarkupMismatch(<div>Text</div>, <div>Other Text</div>));

src/renderers/dom/shared/dangerousStyleValue.js

@@ -22,10 +22,9 @@ var isUnitlessNumber = CSSProperty.isUnitlessNumber;
  *
  * @param {string} name CSS property name such as `topMargin`.
  * @param {*} value CSS property value such as `10px`.
- * @param {ReactDOMComponent} component
  * @return {string} Normalized style value with dimensions applied.
  */
-function dangerousStyleValue(name, value, component, isCustomProperty) {
+function dangerousStyleValue(name, value, isCustomProperty) {
   // Note that we've removed escapeTextForBrowser() calls here since the
   // whole string will be escaped when the attribute is injected into
   // the markup. If you provide unsafe user data here they can inject