diff --git a/src/components/inputs/StringInput.jsx b/src/components/inputs/StringInput.jsx
index e160d69..87bdd31 100644
--- a/src/components/inputs/StringInput.jsx
+++ b/src/components/inputs/StringInput.jsx
@@ -8,10 +8,15 @@ class StringInput extends React.Component {
     style: PropTypes.object,
     default: PropTypes.string,
     onChange: PropTypes.func,
+    onInput: PropTypes.func,
     multi: PropTypes.bool,
     required: PropTypes.bool,
   }
 
+  static defaultProps = {
+    onInput: () => {},
+  }
+
   constructor(props) {
     super(props)
     this.state = {
@@ -57,7 +62,9 @@ class StringInput extends React.Component {
         this.setState({
           editing: true,
           value: e.target.value
-        })
+        }, () => {
+          this.props.onInput(this.state.value);
+        });
       },
       onBlur: () => {
         if(this.state.value!==this.props.value) {
diff --git a/src/components/inputs/UrlInput.jsx b/src/components/inputs/UrlInput.jsx
new file mode 100644
index 0000000..335d3ff
--- /dev/null
+++ b/src/components/inputs/UrlInput.jsx
@@ -0,0 +1,77 @@
+import React from 'react'
+import PropTypes from 'prop-types'
+import StringInput from './StringInput'
+import SmallError from '../util/SmallError'
+
+
+function validate (url) {
+  let error;
+  const getProtocol = (url) => {
+    try {
+      const urlObj = new URL(url);
+      return urlObj.protocol;
+    }
+    catch (err) {
+      return undefined;
+    }
+  };
+  const protocol = getProtocol(url);
+  if (
+    protocol &&
+    protocol === "http:" &&
+    window.location.protocol === "https:"
+  ) {
+    error = (
+      <SmallError>
+        CORS policy won&apos;t allow fetching resources served over http from https, use a <code>https://</code> domain
+      </SmallError>
+    );
+  }
+
+  return error;
+}
+
+class UrlInput extends React.Component {
+  static propTypes = {
+    "data-wd-key": PropTypes.string,
+    value: PropTypes.string,
+    style: PropTypes.object,
+    default: PropTypes.string,
+    onChange: PropTypes.func,
+    onInput: PropTypes.func,
+    multi: PropTypes.bool,
+    required: PropTypes.bool,
+  }
+
+  static defaultProps = {
+    onInput: () => {},
+  }
+
+  constructor (props) {
+    super(props);
+    this.state = {
+      error: validate(props.value)
+    };
+  }
+
+  onInput = (url) => {
+    this.setState({
+      error: validate(url)
+    });
+    this.props.onInput(url);
+  }
+
+  render () {
+    return (
+      <div>
+        <StringInput
+          {...this.props}
+          onInput={this.onInput}
+        />
+        {this.state.error}
+      </div>
+    );
+  }
+}
+
+export default UrlInput
diff --git a/src/components/map/MapboxGlMap.jsx b/src/components/map/MapboxGlMap.jsx
index 87ee61b..7047d6a 100644
--- a/src/components/map/MapboxGlMap.jsx
+++ b/src/components/map/MapboxGlMap.jsx
@@ -181,6 +181,10 @@ export default class MapboxGlMap extends React.Component {
       })
     })
 
+    map.on("error", e => {
+      console.log("ERROR", e);
+    })
+
     map.on("zoom", e => {
       this.setState({
         zoom: map.getZoom()
diff --git a/src/components/modals/OpenModal.jsx b/src/components/modals/OpenModal.jsx
index d2c6477..67f289d 100644
--- a/src/components/modals/OpenModal.jsx
+++ b/src/components/modals/OpenModal.jsx
@@ -4,6 +4,7 @@ import LoadingModal from './LoadingModal'
 import Modal from './Modal'
 import Button from '../Button'
 import FileReaderInput from 'react-file-reader-input'
+import UrlInput from '../inputs/UrlInput'
 
 import {MdFileUpload} from 'react-icons/md'
 import {MdAddCircleOutline} from 'react-icons/md'
@@ -122,9 +123,8 @@ class OpenModal extends React.Component {
     })
   }
 
-  onOpenUrl = () => {
-    const url = this.styleUrlElement.value;
-    this.onStyleSelect(url);
+  onOpenUrl = (url) => {
+    this.onStyleSelect(this.state.styleUrl);
   }
 
   onUpload = (_, files) => {
@@ -160,9 +160,9 @@ class OpenModal extends React.Component {
     this.props.onOpenToggle();
   }
 
-  onChangeUrl = () => {
+  onChangeUrl = (url) => {
     this.setState({
-      styleUrl: this.styleUrlElement.value
+      styleUrl: url,
     });
   }
 
@@ -209,14 +209,13 @@ class OpenModal extends React.Component {
             <p>
               Load from a URL. Note that the URL must have <a href="https://enable-cors.org" target="_blank" rel="noopener noreferrer">CORS enabled</a>.
             </p>
-            <input
+            <UrlInput
               data-wd-key="open-modal.url.input"
               type="text"
-              ref={(input) => this.styleUrlElement = input}
               className="maputnik-input"
-              placeholder="Enter URL..."
+              default="Enter URL..."
               value={this.state.styleUrl}
-              onChange={this.onChangeUrl}
+              onInput={this.onChangeUrl}
             />
             <div>
               <Button
diff --git a/src/components/modals/SettingsModal.jsx b/src/components/modals/SettingsModal.jsx
index de4ced5..4316317 100644
--- a/src/components/modals/SettingsModal.jsx
+++ b/src/components/modals/SettingsModal.jsx
@@ -6,6 +6,7 @@ import InputBlock from '../inputs/InputBlock'
 import ArrayInput from '../inputs/ArrayInput'
 import NumberInput from '../inputs/NumberInput'
 import StringInput from '../inputs/StringInput'
+import UrlInput from '../inputs/UrlInput'
 import SelectInput from '../inputs/SelectInput'
 import EnumInput from '../inputs/EnumInput'
 import ColorField from '../fields/ColorField'
@@ -84,7 +85,7 @@ class SettingsModal extends React.Component {
       onOpenToggle={this.props.onOpenToggle}
       title={'Style Settings'}
     >
-      <div style={{minWidth: 350}}>
+      <div className="modal-settings">
       <InputBlock label={"Name"} doc={latest.$root.name.doc}>
         <StringInput {...inputProps}
           data-wd-key="modal-settings.name" 
@@ -100,7 +101,7 @@ class SettingsModal extends React.Component {
         />
       </InputBlock>
       <InputBlock label={"Sprite URL"} doc={latest.$root.sprite.doc}>
-        <StringInput {...inputProps}
+        <UrlInput {...inputProps}
           data-wd-key="modal-settings.sprite" 
           value={this.props.mapStyle.sprite}
           onChange={this.changeStyleProperty.bind(this, "sprite")}
@@ -108,7 +109,7 @@ class SettingsModal extends React.Component {
       </InputBlock>
 
       <InputBlock label={"Glyphs URL"} doc={latest.$root.glyphs.doc}>
-        <StringInput {...inputProps}
+        <UrlInput {...inputProps}
           data-wd-key="modal-settings.glyphs" 
           value={this.props.mapStyle.glyphs}
           onChange={this.changeStyleProperty.bind(this, "glyphs")}
diff --git a/src/components/modals/SourcesModal.jsx b/src/components/modals/SourcesModal.jsx
index 2d5c8ba..e291607 100644
--- a/src/components/modals/SourcesModal.jsx
+++ b/src/components/modals/SourcesModal.jsx
@@ -112,10 +112,12 @@ class AddSource extends React.Component {
 
   defaultSource(mode) {
     const source = (this.state || {}).source || {}
+    const {protocol} = window.location;
+
     switch(mode) {
       case 'geojson_url': return {
         type: 'geojson',
-        data: 'http://localhost:3000/geojson.json'
+        data: `${protocol}//localhost:3000/geojson.json`
       }
       case 'geojson_json': return {
         type: 'geojson',
@@ -123,31 +125,31 @@ class AddSource extends React.Component {
       }
       case 'tilejson_vector': return {
         type: 'vector',
-        url: source.url || 'http://localhost:3000/tilejson.json'
+        url: source.url || `${protocol}//localhost:3000/tilejson.json`
       }
       case 'tilexyz_vector': return {
         type: 'vector',
-        tiles: source.tiles || ['http://localhost:3000/{x}/{y}/{z}.pbf'],
+        tiles: source.tiles || [`${protocol}//localhost:3000/{x}/{y}/{z}.pbf`],
         minZoom: source.minzoom || 0,
         maxZoom: source.maxzoom || 14
       }
       case 'tilejson_raster': return {
         type: 'raster',
-        url: source.url || 'http://localhost:3000/tilejson.json'
+        url: source.url || `${protocol}//localhost:3000/tilejson.json`
       }
       case 'tilexyz_raster': return {
         type: 'raster',
-        tiles: source.tiles || ['http://localhost:3000/{x}/{y}/{z}.pbf'],
+        tiles: source.tiles || [`${protocol}//localhost:3000/{x}/{y}/{z}.pbf`],
         minzoom: source.minzoom || 0,
         maxzoom: source.maxzoom || 14
       }
       case 'tilejson_raster-dem': return {
         type: 'raster-dem',
-        url: source.url || 'http://localhost:3000/tilejson.json'
+        url: source.url || `${protocol}//localhost:3000/tilejson.json`
       }
       case 'tilexyz_raster-dem': return {
         type: 'raster-dem',
-        tiles: source.tiles || ['http://localhost:3000/{x}/{y}/{z}.pbf'],
+        tiles: source.tiles || [`${protocol}//localhost:3000/{x}/{y}/{z}.pbf`],
         minzoom: source.minzoom || 0,
         maxzoom: source.maxzoom || 14
       }
@@ -155,6 +157,15 @@ class AddSource extends React.Component {
     }
   }
 
+  onAdd = () => {
+    const {source, sourceId} = this.state;
+    this.props.onAdd(sourceId, source);
+  }
+
+  onChangeSource = (source) => {
+    this.setState({source});
+  }
+
   render() {
     return <div className="maputnik-add-source">
       <InputBlock label={"Source ID"} doc={"Unique ID that identifies the source and is used in the layer to reference the source."}>
@@ -180,13 +191,14 @@ class AddSource extends React.Component {
         />
       </InputBlock>
       <SourceTypeEditor
-        onChange={src => this.setState({ source: src })}
+        onChange={this.onChangeSource}
         mode={this.state.mode}
         source={this.state.source}
       />
       <Button
         className="maputnik-add-source-button"
-				onClick={() => this.props.onAdd(this.state.sourceId, this.state.source)}>
+				onClick={this.onAdd}
+      >
         Add Source
       </Button>
     </div>
diff --git a/src/components/sources/SourceTypeEditor.jsx b/src/components/sources/SourceTypeEditor.jsx
index 83672e1..8ed4672 100644
--- a/src/components/sources/SourceTypeEditor.jsx
+++ b/src/components/sources/SourceTypeEditor.jsx
@@ -3,6 +3,7 @@ import PropTypes from 'prop-types'
 import {latest} from '@mapbox/mapbox-gl-style-spec'
 import InputBlock from '../inputs/InputBlock'
 import StringInput from '../inputs/StringInput'
+import UrlInput from '../inputs/UrlInput'
 import NumberInput from '../inputs/NumberInput'
 import SelectInput from '../inputs/SelectInput'
 import JSONEditor from '../layers/JSONEditor'
@@ -18,7 +19,7 @@ class TileJSONSourceEditor extends React.Component {
   render() {
     return <div>
       <InputBlock label={"TileJSON URL"} doc={latest.source_vector.url.doc}>
-        <StringInput
+        <UrlInput
           value={this.props.source.url}
           onChange={url => this.props.onChange({
             ...this.props.source,
@@ -52,7 +53,7 @@ class TileURLSourceEditor extends React.Component {
     const tiles = this.props.source.tiles || []
     return tiles.map((tileUrl, tileIndex) => {
       return <InputBlock key={tileIndex} label={prefix[tileIndex] + " Tile URL"} doc={latest.source_vector.tiles.doc}>
-        <StringInput
+        <UrlInput
           value={tileUrl}
           onChange={this.changeTileUrl.bind(this, tileIndex)}
         />
@@ -95,7 +96,7 @@ class GeoJSONSourceUrlEditor extends React.Component {
 
   render() {
     return <InputBlock label={"GeoJSON URL"} doc={latest.source_geojson.data.doc}>
-      <StringInput
+      <UrlInput
         value={this.props.source.data}
         onChange={data => this.props.onChange({
           ...this.props.source,
diff --git a/src/components/util/SmallError.jsx b/src/components/util/SmallError.jsx
new file mode 100644
index 0000000..03d9c78
--- /dev/null
+++ b/src/components/util/SmallError.jsx
@@ -0,0 +1,20 @@
+import React from 'react'
+import PropTypes from 'prop-types'
+import './SmallError.scss';
+
+
+class SmallError extends React.Component {
+  static propTypes = {
+    children: PropTypes.node,
+  }
+
+  render () {
+    return (
+      <div className="SmallError">
+        Error: {this.props.children}
+      </div>
+    );
+  }
+}
+
+export default SmallError
diff --git a/src/components/util/SmallError.scss b/src/components/util/SmallError.scss
new file mode 100644
index 0000000..1111282
--- /dev/null
+++ b/src/components/util/SmallError.scss
@@ -0,0 +1,7 @@
+@import '../../styles/vars';
+
+.SmallError {
+  color: #E57373;
+  font-size: $font-size-6;
+  margin-top: $margin-2
+}
diff --git a/src/styles/_modal.scss b/src/styles/_modal.scss
index 2f6b1b7..f15cf79 100644
--- a/src/styles/_modal.scss
+++ b/src/styles/_modal.scss
@@ -280,3 +280,7 @@
   color: $color-green;
   margin-top: 16px;
 }
+
+.modal-settings {
+  width: 400px;
+}
diff --git a/src/styles/_vars.scss b/src/styles/_vars.scss
new file mode 100644
index 0000000..7cb3d17
--- /dev/null
+++ b/src/styles/_vars.scss
@@ -0,0 +1,23 @@
+$color-black: #191b20;
+$color-gray: #222429;
+$color-midgray: #303237;
+$color-lowgray: #a4a4a4;
+$color-white: #f0f0f0;
+$color-red: #cf4a4a;
+$color-green: #53b972;
+$margin-1: 3px;
+$margin-2: 5px;
+$margin-3: 10px;
+$margin-4: 30px;
+$margin-5: 40px;
+$font-size-1: 24px;
+$font-size-2: 20px;
+$font-size-3: 18px;
+$font-size-4: 16px;
+$font-size-5: 14px;
+$font-size-6: 12px;
+$font-family: Roboto, sans-serif;
+
+$toolbar-height: 40px;
+$toolbar-offset: 0;
+
diff --git a/src/styles/index.scss b/src/styles/index.scss
index 49af915..759b73c 100644
--- a/src/styles/index.scss
+++ b/src/styles/index.scss
@@ -1,26 +1,4 @@
-$color-black: #191b20;
-$color-gray: #222429;
-$color-midgray: #303237;
-$color-lowgray: #a4a4a4;
-$color-white: #f0f0f0;
-$color-red: #cf4a4a;
-$color-green: #53b972;
-$margin-1: 3px;
-$margin-2: 5px;
-$margin-3: 10px;
-$margin-4: 30px;
-$margin-5: 40px;
-$font-size-1: 24px;
-$font-size-2: 20px;
-$font-size-3: 18px;
-$font-size-4: 16px;
-$font-size-5: 14px;
-$font-size-6: 12px;
-$font-family: Roboto, sans-serif;
-
-$toolbar-height: 40px;
-$toolbar-offset: 0;
-
+@import 'vars';
 @import 'mixins';
 @import 'reset';
 @import 'base';