--- a/devtools/client/inspector/boxmodel/components/BoxModelMain.js
+++ b/devtools/client/inspector/boxmodel/components/BoxModelMain.js
@@ -75,19 +75,28 @@ module.exports = createClass({
   },
 
   getMarginValue(property, direction) {
     let { layout } = this.props.boxModel;
     let autoMargins = layout.autoMargins || {};
     let value = "-";
 
     if (direction in autoMargins) {
-      value = "auto";
+      value = autoMargins[direction];
     } else if (layout[property]) {
-      value = parseFloat(layout[property]);
+      let parsedValue = parseFloat(layout[property]);
+
+      if (Number.isNaN(parsedValue)) {
+        // Not a number. We use the raw string.
+        // Useful for pseudo-elements with auto margins since they
+        // don't appear in autoMargins.
+        value = layout[property];
+      } else {
+        value = parsedValue;
+      }
     }
 
     return value;
   },
 
   getPositionValue(property) {
     let { layout } = this.props.boxModel;
 

--- a/devtools/client/inspector/boxmodel/test/browser.ini
+++ b/devtools/client/inspector/boxmodel/test/browser.ini
@@ -19,16 +19,17 @@ support-files =
 [browser_boxmodel_editablemodel_bluronclick.js]
 [browser_boxmodel_editablemodel_border.js]
 [browser_boxmodel_editablemodel_stylerules.js]
 [browser_boxmodel_guides.js]
 [browser_boxmodel_navigation.js]
 skip-if = true # Bug 1336198
 [browser_boxmodel_offsetparent.js]
 [browser_boxmodel_properties.js]
+[browser_boxmodel_pseudo-element.js]
 [browser_boxmodel_rotate-labels-on-sides.js]
 [browser_boxmodel_sync.js]
 [browser_boxmodel_tooltips.js]
 skip-if = true # Bug 1336198
 [browser_boxmodel_update-after-navigation.js]
 [browser_boxmodel_update-after-reload.js]
 # [browser_boxmodel_update-in-iframes.js]
 # Bug 1020038 boxmodel-view updates for iframe elements changes

new file mode 100644
--- /dev/null
+++ b/devtools/client/inspector/boxmodel/test/browser_boxmodel_pseudo-element.js
@@ -0,0 +1,121 @@
+/* vim: set ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+ http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test that the box model displays the right values for a pseduo-element.
+
+const TEST_URI = `
+  <style type='text/css'>
+    div {
+      box-sizing: border-box;
+      display: block;
+      float: left;
+      line-height: 20px;
+      position: relative;
+      z-index: 2;
+      height: 100px;
+      width: 100px;
+      border: 10px solid black;
+      padding: 20px;
+      margin: 30px auto;
+    }
+
+    div::before {
+      content: 'before';
+      display: block;
+      width: 32px;
+      height: 32px;
+      margin: 0 auto 6px;
+    }
+  </style>
+  <div>Test Node</div>
+`;
+
+// Expected values:
+const res1 = [
+  {
+    selector: ".boxmodel-element-size",
+    value: "32" + "\u00D7" + "32"
+  },
+  {
+    selector: ".boxmodel-size > .boxmodel-width",
+    value: "32"
+  },
+  {
+    selector: ".boxmodel-size > .boxmodel-height",
+    value: "32"
+  },
+  {
+    selector: ".boxmodel-margin.boxmodel-top > span",
+    value: 0
+  },
+  {
+    selector: ".boxmodel-margin.boxmodel-left > span",
+    value: "auto"
+  },
+  {
+    selector: ".boxmodel-margin.boxmodel-bottom > span",
+    value: 6
+  },
+  {
+    selector: ".boxmodel-margin.boxmodel-right > span",
+    value: "auto"
+  },
+  {
+    selector: ".boxmodel-padding.boxmodel-top > span",
+    value: 0
+  },
+  {
+    selector: ".boxmodel-padding.boxmodel-left > span",
+    value: 0
+  },
+  {
+    selector: ".boxmodel-padding.boxmodel-bottom > span",
+    value: 0
+  },
+  {
+    selector: ".boxmodel-padding.boxmodel-right > span",
+    value: 0
+  },
+  {
+    selector: ".boxmodel-border.boxmodel-top > span",
+    value: 0
+  },
+  {
+    selector: ".boxmodel-border.boxmodel-left > span",
+    value: 0
+  },
+  {
+    selector: ".boxmodel-border.boxmodel-bottom > span",
+    value: 0
+  },
+  {
+    selector: ".boxmodel-border.boxmodel-right > span",
+    value: 0
+  },
+];
+
+
+add_task(function* () {
+  yield addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
+  let {inspector, view, testActor} = yield openBoxModelView();
+  let node = yield getNodeFront("div", inspector);
+  let children = yield inspector.markup.walker.children(node);
+  let beforeElement = children.nodes[0];
+
+  yield selectNode(beforeElement, inspector);
+  yield testInitialValues(inspector, view);
+});
+
+function* testInitialValues(inspector, view) {
+  info("Test that the initial values of the box model are correct");
+  let viewdoc = view.document;
+
+  for (let i = 0; i < res1.length; i++) {
+    let elt = viewdoc.querySelector(res1[i].selector);
+    is(elt.textContent, res1[i].value,
+       res1[i].selector + " has the right value.");
+  }
+}