Aquí está el código que RECIBÍ DE W3SCHOOLS. Todo funciona, excepto que en la primera carga de página me da una página en blanco, pero luego, cuando hago clic en uno de los temas, funciona.

 function filterSelection(c) { var x, i; x = document.getElementsByClassName("filterDiv"); if (c == "all") c = ""; for (i = 0; i < x.length; i++) { w3RemoveClass(x[i], "show"); if (x[i].className.indexOf(c) > -1) w3AddClass(x[i], "show"); } } function w3AddClass(element, name) { var i, arr1, arr2; arr1 = element.className.split(" "); arr2 = name.split(" "); for (i = 0; i < arr2.length; i++) { if (arr1.indexOf(arr2[i]) == -1) {element.className += " " + arr2[i];} } } function w3RemoveClass(element, name) { var i, arr1, arr2; arr1 = element.className.split(" "); arr2 = name.split(" "); for (i = 0; i < arr2.length; i++) { while (arr1.indexOf(arr2[i]) > -1) { arr1.splice(arr1.indexOf(arr2[i]), 1); } } element.className = arr1.join(" "); } // Add active class to the current button (highlight it) var btnContainer = document.getElementById("myBtnContainer"); var btns = btnContainer.getElementsByClassName("btn"); for (var i = 0; i < btns.length; i++) { btns[i].addEventListener("click", function(){ var current = document.getElementsByClassName("active"); current[0].className = current[0].className.replace(" active", ""); this.className += " active"; }); }```