[css-syntax-3][cssom-1] Return declarations in specified order

[cdoublev]

The attribute change steps for Element.style, and the procedure for creating a new CSSStyleDeclaration, are receiving declarations from parse a CSS declaration block, in undefined order.

https://drafts.csswg.org/cssom-1/#css-declaration-blocks

A CSS declaration block has these attribute change steps [...]:

[...]

5. Otherwise, let the declarations be the result of parse a CSS declaration block from a string value.

When a CSS declaration block object is created, then:

[...]

If value is not null, let the declarations be the result of parse a CSS declaration block from a string value.

Declarations should be in specified order, which I propose to apply in parse a CSS declaration block.

[cdoublev]

Actually consume a block's contents (CSS Syntax) now filters invalid declarations in the context, so it should also be responsible of applying the specified order, and the steps of parse a CSS declaration block should only be returning the declarations from the result of parse a block's contents.

(edit: fixed) I do not know how to create a link to specified order in CSSOM from CSS Syntax, so I mark this PR as a draft.