They may appear in any order. Tags are case-insensitive . You can write them in small letters, big letters, or any mixture. A common convention is to write them in caps so they stand out from the rest of the document. 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 about which tags you need to put into a web page.