The emphasis is on writing clean, legible, browser-independent HTML, that will be easy to maintain and upgrade. The first word between the angle brackets is the tag's name. A tag is therefore the basic 'item', and an attribute is some extra detail such as how to align the content. You can write them in small letters, big letters, or any mixture. White space and line breaks will not affect the document appearance in a browser except when used inside certain special tags which we'll describe later.
Some people find HTML can be hard to read.
This need not be so if it's written tidily. My own preference is to indent the text by one tab, so that the source has a left margin. Structural tags can then be placed in the margin, and it's easy to read the source. Look at the source of this page to see what I mean. Browsers allow a great deal of flexibility in HTML code, doing their best to try and figure out what to do even when the code isn't technically correct.