Repository: GoogleChrome/lighthouse (js)

Issue:
SEO Score not affected by img without alt attribute
#### Provide the steps to reproduce
1. Run LH on https://www.w3schools.com/code/tryit.asp?filename=G37S02M71XEE . This page contains this code

```
<!DOCTYPE html>
<html>
<body>

<p>
An image that is a link:
<a href="https://www.w3schools.com">
<img src="smiley.gif" width="42" height="42" border="0">
</a>
</p>

</body>
</html>
```

2. Search for SEO score
3. No warning about the missing alt attribute 

#### What is the current behavior?

LH say nothing about img without alt tag

#### What is the expected behavior?

LH should warn about img without alt tag . Google in this document https://support.google.com/webmasters/answer/114016?hl=en says to use descriptive alt text

#### Environment Information
* Affected Channels: DevTools 
* Lighthouse version: 4.0.0
* Node.js version: 6.4.1
* Operating System: updated windows 10



Pull request description:
core(seo): add image alt to SEO category
<!-- Thank you for submitting a pull request! -->

**Summary**
Seems like a good change to sweep in with breaking changes of a major version :)

**Related Issues/PRs**
fixes #8393 


Patch:
diff --git a/lighthouse-core/config/default-config.js b/lighthouse-core/config/default-config.js
index 86dc25925..fa8ff1f05 100644
--- a/lighthouse-core/config/default-config.js
+++ b/lighthouse-core/config/default-config.js
@@ -475,6 +475,7 @@ const defaultConfig = {
         {id: 'link-text', weight: 1, group: 'seo-content'},
         {id: 'is-crawlable', weight: 1, group: 'seo-crawl'},
         {id: 'robots-txt', weight: 1, group: 'seo-crawl'},
+        {id: 'image-alt', weight: 1, group: 'seo-content'},
         {id: 'hreflang', weight: 1, group: 'seo-content'},
         {id: 'canonical', weight: 1, group: 'seo-content'},
         {id: 'font-size', weight: 1, group: 'seo-mobile'},
diff --git a/proto/sample_v2_round_trip.json b/proto/sample_v2_round_trip.json
index e9a1122bf..53783bfa9 100644
--- a/proto/sample_v2_round_trip.json
+++ b/proto/sample_v2_round_trip.json
@@ -3478,6 +3478,11 @@
                     "id": "robots-txt", 
                     "weight": 0.0
                 }, 
+                {
+                    "group": "seo-content", 
+                    "id": "image-alt", 
+                    "weight": 1.0
+                }, 
                 {
                     "group": "seo-content", 
                     "id": "hreflang", 
@@ -3511,7 +3516,7 @@
             "description": "These checks ensure that your page is optimized for search engine results ranking. There are additional factors Lighthouse does not check that may affect your search ranking. [Learn more](https://support.google.com/webmasters/answer/35769).", 
             "id": "seo", 
             "manualDescription": "Run these additional validators on your site to check additional SEO best practices.", 
-            "score": 0.9, 
+            "score": 0.82, 
             "title": "SEO"
         }
     }, 


Test patch:
diff --git a/lighthouse-cli/test/cli/__snapshots__/index-test.js.snap b/lighthouse-cli/test/cli/__snapshots__/index-test.js.snap
index aeaddcff2..3e2964fdf 100644
--- a/lighthouse-cli/test/cli/__snapshots__/index-test.js.snap
+++ b/lighthouse-cli/test/cli/__snapshots__/index-test.js.snap
@@ -962,6 +962,11 @@ Object {
           "id": "robots-txt",
           "weight": 1,
         },
+        Object {
+          "group": "seo-content",
+          "id": "image-alt",
+          "weight": 1,
+        },
         Object {
           "group": "seo-content",
           "id": "hreflang",
diff --git a/lighthouse-core/test/results/sample_v2.json b/lighthouse-core/test/results/sample_v2.json
index c7ed7e658..b9e493af3 100644
--- a/lighthouse-core/test/results/sample_v2.json
+++ b/lighthouse-core/test/results/sample_v2.json
@@ -3491,6 +3491,11 @@
           "weight": 0,
           "group": "seo-crawl"
         },
+        {
+          "id": "image-alt",
+          "weight": 1,
+          "group": "seo-content"
+        },
         {
           "id": "hreflang",
           "weight": 1,
@@ -3522,7 +3527,7 @@
         }
       ],
       "id": "seo",
-      "score": 0.9
+      "score": 0.82
     },
     "pwa": {
       "title": "Progressive Web App",
@@ -5438,4 +5443,4 @@
     }
   },
   "stackPacks": []
-}
+}
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