问题:

Are they allowed? and do they work with all browsers?

Example:

<div role = "region"
     id = "some-id"
     class = "a-class another-class">

回答1:

Yes, any amount of whitespace is allowed and will work in all browsers.

One consideration - this will add to the page size, so if bandwidth and performance are concerns, try to limit the amount of whitespace you use.

回答2:

Yes, it is perfectly valid markup. Whitespace is handled nicely by all browsers.

Any time you have confusion, you can validate your code at official W3 validation service:

http://validator.w3.org/

回答3:

Yes they are, and they will work in all major browsers, although I would say it should be considered bad practice to include unnecessary white-space as it pointlessly increases the size of the document.

HTML, XHTML, XML and others are all variants of SGML, so if you want to know what is/isn't allowed in general, have a look at that specification. You should always pass all your documents through the W3C markup validators to ensure they are valid.