diff --git a/_locales/en/messages.json b/_locales/en/messages.json
index b1cd5297..ae1f59bd 100644
--- a/_locales/en/messages.json
+++ b/_locales/en/messages.json
@@ -583,6 +583,18 @@
     "message": "Update failed - server unreachable.",
     "description": "Text that displays when an update check failed because the update server is unreachable"
   },
+  "updateCheckSkippedLocallyEdited": {
+    "message": "This style was edited locally.",
+    "description": "Text that displays when an update check skipped updating the style to avoid losing local modifications"
+  },
+  "updateCheckSkippedMaybeLocallyEdited": {
+    "message": "This style might have been edited locally.",
+    "description": "Text that displays when an update check skipped updating the style to avoid losing possible local modifications"
+  },
+  "updateCheckManualUpdateHint": {
+    "message": "Do a one-time manual update on its userstyles.org page (your edits will be lost)",
+    "description": "Additional text displayed when an update check skipped updating the style to avoid losing local modifications"
+  },
   "updateCheckSucceededNoUpdate": {
     "message": "Style is up to date.",
     "description": "Text that displays when an update check completed and no update is available"
diff --git a/install.js b/install.js
index 770758a4..9923bad3 100644
--- a/install.js
+++ b/install.js
@@ -104,7 +104,10 @@ function saveStyleCode(message, name, addProps) {
     }
     getResource(getMeta('stylish-code-chrome')).then(code => {
       chrome.runtime.sendMessage(
-        Object.assign(JSON.parse(code), addProps, {method: 'saveStyle'}),
+        Object.assign(JSON.parse(code), addProps, {
+          method: 'saveStyle',
+          reason: 'update',
+        }),
         () => sendEvent('styleInstalledChrome')
       );
       resolve();
diff --git a/manage.js b/manage.js
index f1a42779..cfb04e66 100644
--- a/manage.js
+++ b/manage.js
@@ -573,6 +573,10 @@ function reportUpdateState(state, style, details) {
         details = t('updateCheckFailServerUnreachable');
       } else if (typeof details == 'number') {
         details = t('updateCheckFailBadResponseCode', [details]);
+      } else if (details == BG.updater.SKIPPED_EDITED) {
+        details = t('updateCheckSkippedLocallyEdited') + '\n' + t('updateCheckManualUpdateHint');
+      } else if (details == BG.updater.SKIPPED_MAYBE_EDITED) {
+        details = t('updateCheckSkippedMaybeLocallyEdited') + '\n' + t('updateCheckManualUpdateHint');
       }
       const same =
         details == BG.updater.SKIPPED_SAME_MD5 ||
diff --git a/storage.js b/storage.js
index 2ea84ecc..a98e95f1 100644
--- a/storage.js
+++ b/storage.js
@@ -5,6 +5,7 @@ const RX_NAMESPACE = new RegExp([/[\s\r\n]*/,
   /[\s\r\n]*/].map(rx => rx.source).join(''), 'g');
 const RX_CSS_COMMENTS = /\/\*[\s\S]*?\*\//g;
 const SLOPPY_REGEXP_PREFIX = '\0';
+const DIGEST_KEY_PREFIX = 'originalDigest';
 
 // Note, only 'var'-declared variables are visible from another extension page
 // eslint-disable-next-line no-var
@@ -21,6 +22,26 @@ var cachedStyles = {
   },
 };
 
+// eslint-disable-next-line no-var
+var chromeLocal = {
+  get(options) {
+    return new Promise(resolve => {
+      chrome.storage.local.get(options, data => resolve(data));
+    });
+  },
+  set(data) {
+    return new Promise(resolve => {
+      chrome.storage.local.set(data, () => resolve(data));
+    });
+  },
+  getValue(key) {
+    return chromeLocal.get(key).then(data => data[key]);
+  },
+  setValue(key, value) {
+    return chromeLocal.set({[key]: value});
+  },
+};
+
 
 function getDatabase(ready, error) {
   const dbOpenRequest = window.indexedDB.open('stylish', 2);
@@ -214,7 +235,7 @@ function saveStyle(style) {
           const existed = Boolean(eventGet.target.result);
           const oldStyle = Object.assign({}, eventGet.target.result);
           const codeIsUpdated = 'sections' in style && !styleSectionsEqual(style, oldStyle);
-          write(Object.assign(oldStyle, style), {existed, codeIsUpdated});
+          write(Object.assign(oldStyle, style), {reason, existed, codeIsUpdated});
         };
       } else {
         // Create
@@ -226,10 +247,10 @@ function saveStyle(style) {
           md5Url: null,
           url: null,
           originalMd5: null,
-        }, style));
+        }, style), {reason});
       }
 
-      function write(style, {existed, codeIsUpdated} = {}) {
+      function write(style, {reason, existed, codeIsUpdated} = {}) {
         style.sections = (style.sections || []).map(section =>
           Object.assign({
             urls: [],
@@ -248,6 +269,9 @@ function saveStyle(style) {
               style, codeIsUpdated, reason,
             });
           }
+          if (reason == 'update') {
+            updateStyleDigest(style);
+          }
           resolve(style);
         };
       }
@@ -257,6 +281,7 @@ function saveStyle(style) {
 
 
 function deleteStyle({id, notify = true}) {
+  chrome.storage.local.remove(DIGEST_KEY_PREFIX + id, ignoreChromeError);
   return new Promise(resolve =>
     getDatabase(db => {
       const tx = db.transaction(['styles'], 'readwrite');
@@ -507,3 +532,32 @@ function getDomains(url) {
   }
   return domains;
 }
+
+
+function getStyleDigests(style) {
+  return Promise.all([
+    chromeLocal.getValue(DIGEST_KEY_PREFIX + style.id),
+    calcStyleDigest(style),
+  ]);
+}
+
+
+function updateStyleDigest(style) {
+  calcStyleDigest(style).then(digest =>
+    chromeLocal.set({[DIGEST_KEY_PREFIX + style.id]: digest}));
+}
+
+
+function calcStyleDigest({sections}) {
+  const text = new TextEncoder('utf-8').encode(JSON.stringify(sections));
+  return crypto.subtle.digest('SHA-1', text).then(hex);
+  function hex(buffer) {
+    const parts = [];
+    const PAD8 = '00000000';
+    const view = new DataView(buffer);
+    for (let i = 0; i < view.byteLength; i += 4) {
+      parts.push((PAD8 + view.getUint32(i).toString(16)).slice(-8));
+    }
+    return parts.join('');
+  }
+}
diff --git a/update.js b/update.js
index aeee808b..1055a55b 100644
--- a/update.js
+++ b/update.js
@@ -1,4 +1,4 @@
-/* globals getStyles, saveStyle, styleSectionsEqual */
+/* globals getStyles, saveStyle, styleSectionsEqual, getStyleDigests, updateStyleDigest */
 'use strict';
 
 // eslint-disable-next-line no-var
@@ -7,6 +7,8 @@ var updater = {
   COUNT: 'count',
   UPDATED: 'updated',
   SKIPPED: 'skipped',
+  SKIPPED_EDITED: 'locally edited',
+  SKIPPED_MAYBE_EDITED: 'maybe locally edited',
   SKIPPED_SAME_MD5: 'up-to-date: MD5 is unchanged',
   SKIPPED_SAME_CODE: 'up-to-date: code sections are unchanged',
   SKIPPED_ERROR_MD5: 'error: MD5 is invalid',
@@ -32,33 +34,60 @@ var updater = {
   },
 
   checkStyle(style, observe = () => {}, {save = true} = {}) {
-    return download(style.md5Url)
-      .then(md5 =>
-        !md5 || md5.length != 32 ? Promise.reject(updater.SKIPPED_ERROR_MD5) :
-        md5 == style.originalMd5 ? Promise.reject(updater.SKIPPED_SAME_MD5) :
-        style.updateUrl)
-      .then(download)
-      .then(text => tryJSONparse(text))
-      .then(json =>
-        !updater.styleJSONseemsValid(json) ? Promise.reject(updater.SKIPPED_ERROR_JSON) :
-        styleSectionsEqual(json, style) ? Promise.reject(updater.SKIPPED_SAME_CODE) :
-        // keep the local name as it could've been customized by the user
-        Object.assign(json, {
-          id: style.id,
-          name: null,
-          reason: 'update',
-        }))
-      .then(json => save ? saveStyle(json) : json)
+    let hasDigest;
+    return getStyleDigests(style)
+      .then(fetchMd5IfNotEdited)
+      .then(fetchCodeIfMd5Changed)
+      .then(saveIfUpdated)
       .then(saved => observe(updater.UPDATED, saved))
       .catch(err => observe(updater.SKIPPED, style, err));
-  },
 
-  styleJSONseemsValid(json) {
-    return json
-      && json.sections
-      && json.sections.length
-      && typeof json.sections.every == 'function'
-      && typeof json.sections[0].code == 'string';
+    function fetchMd5IfNotEdited([originalDigest, current]) {
+      hasDigest = Boolean(originalDigest);
+      if (hasDigest && originalDigest != current) {
+        return Promise.reject(updater.SKIPPED_EDITED);
+      }
+      return download(style.md5Url);
+    }
+
+    function fetchCodeIfMd5Changed(md5) {
+      if (!md5 || md5.length != 32) {
+        return Promise.reject(updater.SKIPPED_ERROR_MD5);
+      }
+      if (md5 == style.originalMd5 && hasDigest) {
+        return Promise.reject(updater.SKIPPED_SAME_MD5);
+      }
+      return download(style.updateUrl);
+    }
+
+    function saveIfUpdated(text) {
+      const json = tryJSONparse(text);
+      if (!styleJSONseemsValid(json)) {
+        return Promise.reject(updater.SKIPPED_ERROR_JSON);
+      }
+      json.id = style.id;
+      if (styleSectionsEqual(json, style)) {
+        if (!hasDigest) {
+          updateStyleDigest(json);
+        }
+        return Promise.reject(updater.SKIPPED_SAME_CODE);
+      } else if (!hasDigest) {
+        return Promise.reject(updater.SKIPPED_MAYBE_EDITED);
+      }
+      return !save ? json :
+        saveStyle(Object.assign(json, {
+          name: null, // keep local name customizations
+          reason: 'update',
+        }));
+    }
+
+    function styleJSONseemsValid(json) {
+      return json
+        && json.sections
+        && json.sections.length
+        && typeof json.sections.every == 'function'
+        && typeof json.sections[0].code == 'string';
+    }
   },
 
   schedule() {