diff --git a/src/Registration.js b/src/Registration.js
index f86c9cc618..98aee3ac83 100644
--- a/src/Registration.js
+++ b/src/Registration.js
@@ -26,6 +26,10 @@ import MatrixClientPeg from './MatrixClientPeg';
 import Modal from './Modal';
 import { _t } from './languageHandler';
 
+// Regex for what a "safe" or "Matrix-looking" localpart would be.
+// TODO: Update as needed for https://github.com/matrix-org/matrix-doc/issues/1514
+export const SAFE_LOCALPART_REGEX = /^[a-z0-9=_\-./]+$/;
+
 /**
  * Starts either the ILAG or full registration flow, depending
  * on what the HS supports
diff --git a/src/components/structures/login/Registration.js b/src/components/structures/login/Registration.js
index 98fd61370d..ad3ea5f19c 100644
--- a/src/components/structures/login/Registration.js
+++ b/src/components/structures/login/Registration.js
@@ -342,7 +342,7 @@ module.exports = React.createClass({
                 errMsg = _t('A phone number is required to register on this homeserver.');
                 break;
             case "RegistrationForm.ERR_USERNAME_INVALID":
-                errMsg = _t('User names may only contain letters, numbers, dots, hyphens and underscores.');
+                errMsg = _t("Only use lower case letters, numbers and '=_-./'");
                 break;
             case "RegistrationForm.ERR_USERNAME_BLANK":
                 errMsg = _t('You need to enter a user name.');
diff --git a/src/components/views/dialogs/SetMxIdDialog.js b/src/components/views/dialogs/SetMxIdDialog.js
index fb892c4a0a..222a2c35fe 100644
--- a/src/components/views/dialogs/SetMxIdDialog.js
+++ b/src/components/views/dialogs/SetMxIdDialog.js
@@ -23,6 +23,7 @@ import MatrixClientPeg from '../../../MatrixClientPeg';
 import classnames from 'classnames';
 import { KeyCode } from '../../../Keyboard';
 import { _t } from '../../../languageHandler';
+import { SAFE_LOCALPART_REGEX } from '../../../Registration';
 
 // The amount of time to wait for further changes to the input username before
 // sending a request to the server
@@ -110,12 +111,11 @@ export default React.createClass({
     },
 
     _doUsernameCheck: function() {
-        // XXX: SPEC-1
-        // Check if username is valid
-        // Naive impl copied from https://github.com/matrix-org/matrix-react-sdk/blob/66c3a6d9ca695780eb6b662e242e88323053ff33/src/components/views/login/RegistrationForm.js#L190
-        if (encodeURIComponent(this.state.username) !== this.state.username) {
+        // We do a quick check ahead of the username availability API to ensure the
+        // user ID roughly looks okay from a Matrix perspective.
+        if (!SAFE_LOCALPART_REGEX.test(this.state.username)) {
             this.setState({
-                usernameError: _t('User names may only contain letters, numbers, dots, hyphens and underscores.'),
+                usernameError: _t("Only use lower case letters, numbers and '=_-./'"),
             });
             return Promise.reject();
         }
@@ -210,7 +210,6 @@ export default React.createClass({
     render: function() {
         const BaseDialog = sdk.getComponent('views.dialogs.BaseDialog');
         const InteractiveAuth = sdk.getComponent('structures.InteractiveAuth');
-        const Spinner = sdk.getComponent('elements.Spinner');
 
         let auth;
         if (this.state.doingUIAuth) {
@@ -230,9 +229,8 @@ export default React.createClass({
         });
 
         let usernameIndicator = null;
-        let usernameBusyIndicator = null;
         if (this.state.usernameBusy) {
-            usernameBusyIndicator = <Spinner w="24" h="24" />;
+            usernameIndicator = <div>{_t("Checking...")}</div>;
         } else {
             const usernameAvailable = this.state.username &&
                 this.state.usernameCheckSupport && !this.state.usernameError;
@@ -270,7 +268,6 @@ export default React.createClass({
                             size="30"
                             className={inputClasses}
                         />
-                        { usernameBusyIndicator }
                     </div>
                     { usernameIndicator }
                     <p>
diff --git a/src/components/views/login/RegistrationForm.js b/src/components/views/login/RegistrationForm.js
index fe977025ae..137aeada91 100644
--- a/src/components/views/login/RegistrationForm.js
+++ b/src/components/views/login/RegistrationForm.js
@@ -25,7 +25,7 @@ import { looksValid as phoneNumberLooksValid } from '../../../phonenumber';
 import Modal from '../../../Modal';
 import { _t } from '../../../languageHandler';
 import SdkConfig from '../../../SdkConfig';
-import SettingsStore from "../../../settings/SettingsStore";
+import { SAFE_LOCALPART_REGEX } from '../../../Registration';
 
 const FIELD_EMAIL = 'field_email';
 const FIELD_PHONE_COUNTRY = 'field_phone_country';
@@ -194,9 +194,8 @@ module.exports = React.createClass({
                 } else this.markFieldValid(field_id, phoneNumberValid, "RegistrationForm.ERR_PHONE_NUMBER_INVALID");
                 break;
             case FIELD_USERNAME:
-                // XXX: SPEC-1
-                var username = this.refs.username.value.trim();
-                if (encodeURIComponent(username) != username) {
+                const username = this.refs.username.value.trim();
+                if (!SAFE_LOCALPART_REGEX.test(username)) {
                     this.markFieldValid(
                         field_id,
                         false,
diff --git a/src/components/views/room_settings/AliasSettings.js b/src/components/views/room_settings/AliasSettings.js
index de5d3db625..f68670b2f9 100644
--- a/src/components/views/room_settings/AliasSettings.js
+++ b/src/components/views/room_settings/AliasSettings.js
@@ -130,7 +130,7 @@ module.exports = React.createClass({
     },
 
     isAliasValid: function(alias) {
-        // XXX: FIXME SPEC-1
+        // XXX: FIXME https://github.com/matrix-org/matrix-doc/issues/668
         return (alias.match(/^#([^\/:,]+?):(.+)$/) && encodeURI(alias) === alias);
     },
 
diff --git a/src/i18n/strings/en_EN.json b/src/i18n/strings/en_EN.json
index a97182520a..00f781ea5b 100644
--- a/src/i18n/strings/en_EN.json
+++ b/src/i18n/strings/en_EN.json
@@ -985,10 +985,11 @@
     "Unable to verify email address.": "Unable to verify email address.",
     "This will allow you to reset your password and receive notifications.": "This will allow you to reset your password and receive notifications.",
     "Skip": "Skip",
-    "User names may only contain letters, numbers, dots, hyphens and underscores.": "User names may only contain letters, numbers, dots, hyphens and underscores.",
+    "Only use lower case letters, numbers and '=_-./'": "Only use lower case letters, numbers and '=_-./'",
     "Username not available": "Username not available",
     "Username invalid: %(errMessage)s": "Username invalid: %(errMessage)s",
     "An error occurred: %(error_string)s": "An error occurred: %(error_string)s",
+    "Checking...": "Checking...",
     "Username available": "Username available",
     "To get started, please pick a username!": "To get started, please pick a username!",
     "This will be your account name on the <span></span> homeserver, or you can pick a <a>different server</a>.": "This will be your account name on the <span></span> homeserver, or you can pick a <a>different server</a>.",