I only know HTML

honestly, there's no difference between html and xhtml from an author's perspective.

i elborate:

• both should be lower-case
• both should be properly nested (<b><i></i></b> vs <b><i></b></i>)
• everything should be closed (xhtml just introduces self closing tags [<img />] to make this more visbaly logical)
• doctypes must be used - unless the xhtml document is parsed as application/xml+xhtml it renderes it as plain text/html (meaning it's just fancy html now) and the browser will render it in quirks-mode wen no doctype is present. so really, they behave the same way in regards to doctypes.

the design logic is exactly the same, the syntax just got neater and but a little more focus on sticking to webstandards (removing unecessary tags like <font>).