less/mainmenu.less has various css rules related to the main menu.
These should only be loaded on Special:MobileOptions or in javascript as part of the mobile.mainMenu ResourceLoader module.
```
.secondary-navigation-enabled #mw-mf-page-center
.secondary-navigation-enabled #mw-mf-page-center .main-header-button
.notifications-overlay.navigation-drawer
.animations .notifications-overlay.navigation-drawer
.animations .notifications-overlay.navigation-drawer.visible
.navigation-enabled.animations .position-fixed,.navigation-enabled.animations #mw-mf-page-center
.primary-navigation-enabled.animations .position-fixed,.primary-navigation-enabled.animations #mw-mf-page-center
.primary-navigation-enabled.animations nav
.secondary-navigation-enabled.animations #mw-mf-page-center
.rtl .animations .notifications-overlay.navigation-drawer
.rtl .animations .notifications-overlay.navigation-drawer.visible
.rtl.primary-navigation-enabled.animations .position-fixed,.rtl.primary-navigation-enabled.animations #mw-mf-page-center
.rtl.secondary-navigation-enabled.animations #mw-mf-page-center
nav
.transparent-shield,.navigation-drawer
.notifications-overlay.navigation-drawer
nav
.client-js nav .icon-secondary,.client-js nav .secondary-action
.client-js nav .icon-profile,.client-js nav .primary-action
nav ul:first-child
nav ul:first-child li:first-child
nav ul li
nav ul li:hover
nav ul li:hover a
nav ul li:last-child
nav ul li a
nav ul li a:hover
nav ul li a:visited,nav ul li a:active
nav ul.hlist li
nav ul.hlist li a
body.navigation-enabled #mw-mf-viewport
body.navigation-enabled #mw-mf-page-center
body.navigation-enabled .navigation-drawer,body.navigation-enabled .transparent-shield
body.navigation-enabled .transparent-shield
.primary-navigation-enabled
.primary-navigation-enabled #mw-mf-viewport
.primary-navigation-enabled nav
.primary-navigation-enabled .position-fixed
.animations #mw-mf-page-center
.animations nav
```