Yes - both getElementsByClassName and getElementsByTagName retrieve lists and your commands are trying to apply attributes to the whole list that are only allowed to be applied to individual nodes within the list.
To apply to the first node in each list you’d use:
document.getElementsByClassName("navigation")[0].style.height = 20;
document.getElementsByTagName("a")[0].style.fontSize = 12;
To apply to all elements in the list you’d use:
[].forEach.call(document.getElementsByClassName("navigation"), function(el) {
el.style.height = 20;
});
[].forEach.call(document.getElementsByTagName("a"), function(el) {
el.style.fontSize = 12;
});