CSS3 combining selectors with OR instead of AND

You'll need to split them up using a comma:

body[class*="page-node-add-"], body[class~="page-node-edit"] {background:red;}

The problem with using a comma:

... is that you can't do it any other way than with a comma. Perhaps it could have been remedied with Selectors 3, but unfortunately the spec says otherwise. That is only going to be remedied by Selectors 4, either because it wasn't proposed until recently, or it was proposed but didn't make the cut for level 3.

In level 4 of Selectors you will be able to do something like this:

body:matches([class*="page-node-add-"], [class~="page-node-edit"]) form.node-form > .field-type-field-collection > table > thead tr th
{...}

Currently, this is being implemented under its originally-proposed name, :any(), with the prefixes :-moz-any() and :-webkit-any(). But using :any() in public-facing CSS is pointless given that

1. only Gecko and WebKit support it; and

2. you have to duplicate your rulesets because of the way prefixed selectors are handled, which not only defeats the intended purpose of the :matches() selector, but makes things even worse:

   body:-moz-any([class*="page-node-add-"], [class~="page-node-edit"]) form.node-form > .field-type-field-collection > table > thead tr th
   {...}
   body:-webkit-any([class*="page-node-add-"], [class~="page-node-edit"]) form.node-form > .field-type-field-collection > table > thead tr th
   {...}
   

In other words, until implementations update themselves to the standardized :matches(), there is no other viable solution (save from using a preprocessor to generate the repeated selectors for you).

I found the answer here:

CSS Shorthand to identify multiple classes

Mozilla and webkit has a -moz-any or -webkit-any, though in the CSS4 spec there is a :matches. Suprised this wasn't thought of in CSS3 as it would greatly reduce the amount of repetative code without having to use SASS or LESS or whatever.