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<html><head><script src="https://ajax.googleapis.com/ajax/libs/jquery/3.7.1/jquery.min.js"></script><script>$(document).ready(function(){ $("button").click(function(){ $("#div1").addClass("important blue"); });});</script><style>.important { font-weight: bold; font-size: xx-large;}.blue { color: blue;}</style></head><body><div id="div1">This is some text.</div><div id="div2">This is some text.</div><br><button>Add classes to first div element</button></body></html>