diff --git a/lighthouse-core/audits/audit.js b/lighthouse-core/audits/audit.js
index fce625e1e215..6ee2851c18a6 100644
--- a/lighthouse-core/audits/audit.js
+++ b/lighthouse-core/audits/audit.js
@@ -70,7 +70,7 @@ class Audit {
    * Table cells will use the type specified in headings[x].itemType. However a custom type
    * can be provided: results[x].propName = {type: 'code', text: '...'}
    * @param {!Audit.Headings} headings
-   * @param {!Array<!Object<string, string>>} results
+   * @param {!Array<!Object<string, *>>} results
    * @return {!Array<!DetailsRenderer.DetailsJSON>}
    */
   static makeV2TableRows(headings, results) {
@@ -157,13 +157,27 @@ class Audit {
 
 module.exports = Audit;
 
-/** @typedef {{
- *     key: string,
- *     itemType: string,
- *     text: string,
- * }}
+/** @typedef {
+ * !Array<{
+ *   key: string,
+ *   itemType: string,
+ *   text: string,
+ * }>}
  */
-Audit.Heading; // eslint-disable-line no-unused-expressions
-
-/** @typedef {!Array<!Audit.Heading>} */
 Audit.Headings; // eslint-disable-line no-unused-expressions
+
+
+{{results: !Array<Object>, headings: !Audit.Headings, passes: boolean, debugString: (string|undefined)}}
+{{results: !Array<Object>, passes: boolean=, headings: !Audit.Headings, debugString: string=}}
+{{results: !Array<Object>, headings: !Audit.Headings, passes: boolean, debugString: string=}}
+{!{debugString: string=, passes: boolean=, headings: !Audit.Headings, results: !Array<!Object<string, string>>}} result
+
+/** @typedef {
+  * {{
+  *   results: !Array<Object>,
+  *   headings: !Audit.Headings,
+  *   passes: boolean,
+  *   debugString: (string|undefined)
+  * }}
+  */
+  Audit.; // eslint-disable-line no-unused-expressions
\ No newline at end of file
diff --git a/lighthouse-core/audits/byte-efficiency/offscreen-images.js b/lighthouse-core/audits/byte-efficiency/offscreen-images.js
index b3b6907e5b8e..2ea2538f6800 100644
--- a/lighthouse-core/audits/byte-efficiency/offscreen-images.js
+++ b/lighthouse-core/audits/byte-efficiency/offscreen-images.js
@@ -96,7 +96,7 @@ class OffscreenImages extends ByteEfficiencyAudit {
 
   /**
    * @param {!Artifacts} artifacts
-   * @return {{results: !Array<Object>, headings: !Audit.Headings, debugString: string=}}
+   * @return {{results: !Array<Object>, headings: !Audit.Headings, passes: boolean, debugString: (string|undefined)}}
    */
   static audit_(artifacts) {
     const images = artifacts.ImageUsage;
diff --git a/lighthouse-core/audits/byte-efficiency/uses-optimized-images.js b/lighthouse-core/audits/byte-efficiency/uses-optimized-images.js
index bac204a92bc6..4620725b317b 100644
--- a/lighthouse-core/audits/byte-efficiency/uses-optimized-images.js
+++ b/lighthouse-core/audits/byte-efficiency/uses-optimized-images.js
@@ -63,7 +63,7 @@ class UsesOptimizedImages extends ByteEfficiencyAudit {
 
   /**
    * @param {!Artifacts} artifacts
-   * @return {{results: !Array<Object>, headings: !Audit.Headings, passes: boolean=, debugString: string=}}
+   * @return {{results: !Array<Object>, headings: !Audit.Headings, passes: boolean, debugString: (string|undefined)}}
    */
   static audit_(artifacts) {
     const images = artifacts.OptimizedImages;
diff --git a/lighthouse-core/audits/byte-efficiency/uses-request-compression.js b/lighthouse-core/audits/byte-efficiency/uses-request-compression.js
index 2b43cd0fd0b0..3ee0c011a8d8 100644
--- a/lighthouse-core/audits/byte-efficiency/uses-request-compression.js
+++ b/lighthouse-core/audits/byte-efficiency/uses-request-compression.js
@@ -47,7 +47,7 @@ class ResponsesAreCompressed extends ByteEfficiencyAudit {
   /**
    * @param {!Artifacts} artifacts
    * @param {number} networkThroughput
-   * @return {{results: !Array<Object>, passes: boolean=, headings: !Audit.Headings, debugString: string=}}
+   * @return {{results: !Array<Object>, headings: !Audit.Headings, passes: boolean, debugString: (string|undefined)}}
    */
   static audit_(artifacts) {
     const uncompressedResponses = artifacts.ResponseCompression;
diff --git a/lighthouse-core/audits/byte-efficiency/uses-responsive-images.js b/lighthouse-core/audits/byte-efficiency/uses-responsive-images.js
index 9bea788c1e37..71cc894ca410 100644
--- a/lighthouse-core/audits/byte-efficiency/uses-responsive-images.js
+++ b/lighthouse-core/audits/byte-efficiency/uses-responsive-images.js
@@ -81,7 +81,7 @@ class UsesResponsiveImages extends ByteEfficiencyAudit {
 
   /**
    * @param {!Artifacts} artifacts
-   * @return {{results: !Array<Object>, headings: !Audit.Headings, passes: boolean=, debugString: string=}}
+   * @return {{results: !Array<Object>, headings: !Audit.Headings, passes: boolean, debugString: (string|undefined)}}
    */
   static audit_(artifacts) {
     const images = artifacts.ImageUsage;
diff --git a/lighthouse-core/report/v2/renderer/details-renderer.js b/lighthouse-core/report/v2/renderer/details-renderer.js
index 1c8a870e9457..a9469fa5e794 100644
--- a/lighthouse-core/report/v2/renderer/details-renderer.js
+++ b/lighthouse-core/report/v2/renderer/details-renderer.js
@@ -70,17 +70,19 @@ class DetailsRenderer {
   }
 
   /**
+   * Create small thumbnail with scaled down image asset.
+   * If the supplied details doesn't have an image/* mimeType, then an empty span is returned.
    * @param {!DetailsRenderer.ThumbnailDetails} value
    * @return {!Element}
    */
   _renderThumbnail(value) {
-    if (/^image/.test(value.mimeType) === false) {
+    if (!value.mimeType.startsWith('image/')) {
       return this._dom.createElement('span');
     }
 
     const element = this._dom.createElement('img', 'lh-thumbnail');
     element.src = value.url;
-    element.alt = 'Image preview';
+    element.alt = '';
     element.title = value.url;
     return element;
   }