HTML

differences between html and xhtml

In HTML, you can do this:
<meta charset="UTF-8"> and <meta charset="UTF-8" />
In XHTML you have to do this:
<meta charset="UTF-8" />
You can also do this in XHTML:
<custom-tag />
but you can't in HTML.

difference between xhtml and html

<!--- HTML --->
<br>
<!--- XHTML --->
<br />
<!--- HTML --->
Hypertext Markup Language
<!--- XHTML --->
eXtended Hypertext Markup Language
<!--- HTML --->
<BUTTON> HTML is case-insensitive</BUTTON>
<!--- XHTML --->
<button> Strictly lower case </button>