diff --git a/devtools/shared/indentation.js b/devtools/shared/indentation.js
index 3dfd1a088da5..13c80f32254d 100644
--- a/devtools/shared/indentation.js
+++ b/devtools/shared/indentation.js
@@ -22,7 +22,7 @@ const DETECT_INDENT_MAX_LINES = 500;
  */
 function getTabPrefs() {
   const indentWithTabs = !Services.prefs.getBoolPref(EXPAND_TAB);
-  const indentUnit = Services.prefs.getIntPref(TAB_SIZE);
+  const indentUnit = Services.prefs.getIntPref(TAB_SIZE, 2);
   return { indentUnit, indentWithTabs };
 }
 
diff --git a/devtools/shared/inspector/css-logic.js b/devtools/shared/inspector/css-logic.js
index 7ea71fd14b07..8e05995baca1 100644
--- a/devtools/shared/inspector/css-logic.js
+++ b/devtools/shared/inspector/css-logic.js
@@ -412,6 +412,13 @@ function prettifyCSS(text, ruleCount) {
     return token;
   };
 
+  // Get preference of the user regarding what to use for indentation,
+  // spaces or tabs.
+  const tabPrefs = getTabPrefs();
+  const baseIndentString = tabPrefs.indentWithTabs
+    ? TAB_CHARS
+    : SPACE_CHARS.repeat(tabPrefs.indentUnit);
+
   while (true) {
     // Set the initial state.
     startIndex = undefined;
@@ -437,20 +444,16 @@ function prettifyCSS(text, ruleCount) {
       }
     }
 
-    // Get preference of the user regarding what to use for indentation,
-    // spaces or tabs.
-    const tabPrefs = getTabPrefs();
-
     if (isCloseBrace) {
       // Even if the stylesheet contains extra closing braces, the indent level should
       // remain > 0.
       indentLevel = Math.max(0, indentLevel - 1);
+      indent = baseIndentString.repeat(indentLevel);
 
+      // FIXME: This is incorrect and should be fixed in Bug 1839297
       if (tabPrefs.indentWithTabs) {
-        indent = TAB_CHARS.repeat(indentLevel);
         indentOffset = 4 * indentLevel;
       } else {
-        indent = SPACE_CHARS.repeat(indentLevel);
         indentOffset = 1 * indentLevel;
       }
       result = result + indent + "}";
@@ -466,11 +469,14 @@ function prettifyCSS(text, ruleCount) {
         columnOffset++;
       }
       result += "{";
+      indentLevel++;
+      indent = baseIndentString.repeat(indentLevel);
+      indentOffset = indent.length;
+
+      // FIXME: This is incorrect and should be fixed in Bug 1839297
       if (tabPrefs.indentWithTabs) {
-        indent = TAB_CHARS.repeat(++indentLevel);
         indentOffset = 4 * indentLevel;
       } else {
-        indent = SPACE_CHARS.repeat(++indentLevel);
         indentOffset = 1 * indentLevel;
       }
     }
diff --git a/devtools/shared/tests/xpcshell/test_prettifyCSS.js b/devtools/shared/tests/xpcshell/test_prettifyCSS.js
index 3739c074629a..1839c6a253e2 100644
--- a/devtools/shared/tests/xpcshell/test_prettifyCSS.js
+++ b/devtools/shared/tests/xpcshell/test_prettifyCSS.js
@@ -10,6 +10,7 @@ const {
 } = require("resource://devtools/shared/inspector/css-logic.js");
 
 const EXPAND_TAB = "devtools.editor.expandtab";
+const TAB_SIZE = "devtools.editor.tabsize";
 
 const TESTS_TAB_INDENT = [
   {
@@ -59,7 +60,11 @@ const TESTS_SPACE_INDENT = [
   {
     name: "simple test. indent using spaces",
     input: "div { font-family:'Arial Black', Arial, sans-serif; }",
-    expected: ["div {", " font-family:'Arial Black', Arial, sans-serif;", "}"],
+    expected: [
+      "div {",
+      "    font-family:'Arial Black', Arial, sans-serif;",
+      "}",
+    ],
   },
 
   {
@@ -74,14 +79,14 @@ const TESTS_SPACE_INDENT = [
       "\nbody{background:white;}div{font-size:4em;color:red}span{color:green;}\n",
     expected: [
       "body {",
-      " background:white;",
+      "    background:white;",
       "}",
       "div {",
-      " font-size:4em;",
-      " color:red",
+      "    font-size:4em;",
+      "    color:red",
       "}",
       "span {",
-      " color:green;",
+      "    color:green;",
       "}",
     ],
   },
@@ -89,25 +94,33 @@ const TESTS_SPACE_INDENT = [
   {
     name: "leading whitespace. indent using spaces",
     input: "\n    div{color: red;}",
-    expected: ["div {", " color: red;", "}"],
+    expected: ["div {", "    color: red;", "}"],
   },
 
   {
     name: "CSS with extra closing brace. indent using spaces",
     input: "body{margin:0}} div{color:red}",
-    expected: ["body {", " margin:0", "}", "}", "div {", " color:red", "}"],
+    expected: [
+      "body {",
+      "    margin:0",
+      "}",
+      "}",
+      "div {",
+      "    color:red",
+      "}",
+    ],
   },
 
   {
     name: "HTML comments with some whitespace padding",
     input: "  \n\n\t  <!--\n\n\t body {color:red}  \n\n-->   \t\n",
-    expected: ["body {", " color:red", "}"],
+    expected: ["body {", "    color:red", "}"],
   },
 
   {
     name: "HTML comments without whitespace padding",
     input: "<!--body {color:red}-->",
-    expected: ["body {", " color:red", "}"],
+    expected: ["body {", "    color:red", "}"],
   },
 
   {
@@ -117,18 +130,18 @@ const TESTS_SPACE_INDENT = [
       "div, div, input, pre, table {color: blue;}",
     expected: [
       "@media screen, print {",
-      " div,",
-      " span,",
-      " input {",
-      "  color: red;",
-      " }",
+      "    div,",
+      "    span,",
+      "    input {",
+      "        color: red;",
+      "    }",
       "}",
       "div,",
       "div,",
       "input,",
       "pre,",
       "table {",
-      " color: blue;",
+      "    color: blue;",
       "}",
     ],
   },
@@ -136,7 +149,14 @@ const TESTS_SPACE_INDENT = [
   {
     name: "Multiline comment in CSS",
     input: "/*\n * comment\n */\n#example{display:grid;}",
-    expected: ["/*", " * comment", " */", "#example {", " display:grid;", "}"],
+    expected: [
+      "/*",
+      " * comment",
+      " */",
+      "#example {",
+      "    display:grid;",
+      "}",
+    ],
   },
 ];
 
@@ -146,6 +166,8 @@ function run_test() {
   prettifyCSS("");
 
   Services.prefs.setBoolPref(EXPAND_TAB, true);
+  Services.prefs.setIntPref(TAB_SIZE, 4);
+
   for (const test of TESTS_SPACE_INDENT) {
     info(test.name);
 
@@ -169,4 +191,5 @@ function run_test() {
     equal(output, expected, test.name);
   }
   Services.prefs.clearUserPref(EXPAND_TAB);
+  Services.prefs.clearUserPref(TAB_SIZE);
 }