MediaWiki:Mobile.js

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/* All JavaScript here will be loaded for users of the mobile site */
$(document).ready(function() {
    if (mw.config.get('skin') === 'minerva') { // Minerva is the default mobile skin
        $('<div class="mobile-banner">📢 Special message for mobile users!</div>')
            .prependTo('.mw-body');
    }
});