The value of the attributes need not to be enclosed within double inverted commas?

Question

I have tried giving the width 60% without double inverted commas and it worked. So can anyone explain this?

Answer 1

From the W3C HTML5 working draft:
https://www.w3.org/TR/2011/WD-html5-20110525/introduction.html

Down the page, where Attributes are discussed...

"The attribute value can remain unquoted if it doesn't contain spaces or any of " ' ` = < or >. Otherwise, it has to be quoted using either single or double quotes."

[meta]
Note that "spaces" actually refers to more than just the space you're accustomed to typing (ASCII 32); it includes several more.
Search for "The space characters, for the purposes of this specification, are" ... in section 2.5: https://www.w3.org/TR/2011/WD-html5-20110525/common-microsyntaxes.html