There is no practical difference except
- if you validate your page, quotation marks may or may not be
needed to avoid error messages, depending on doctype being used - if you serve the page with an XML content type to browsers (which
is rare and seldom useful), then the quotes are required – otherwise
the page is not displayed at all, just an error message - if the page is otherwise processed with XML tools, the quotes are necessary.
Otherwise, the quotation marks are really needed only if the attribute value contains a space, a line break, an Ascii quotation mark (“), an Ascii apostrophe (‘), a grave accent (`), an equals sign (=), a less than sign (<), or a greater than sign (>). So style = width:20em would work (though it might be seen as somewhat obscure), whereas style = width: 20em would not – due to the space, you would need to write style = "width: 20em".
Many people always write quotation marks around all attribute values, for simplicity. Others think that quotation marks make the code a bit messy, so they omit them when possible.
Quite independently of this, src="www.example.com" means a relative URL reference, not what people expect to mean. You probably meant src="http://www.example.com".