HTML Document conventions.

I've browsed through the HTML Specs and couldn't find information regarding
the use of quotation marks (don't know if that's the right word, I mean: "")
when defining attributes for HTML tags.

Can anyone tell me whether there is any difference between

<img src="some_picture.png" width="10" height="10" alt="Some alternative
text"> and
<img src="some_picture.png" width=10 height=10 alt="Some alternative text">.
Or may I even write
<img src='some_picture.png' width='10' height='10' alt='Some alternative
text'>

(I've never tried the last one, though)...

Thanks in advance,
--- Skult