How to detect strictly adjacent siblings

Consider the following HTML where IDs #p1, #p2 and #p3 are siblings (see fiddle):

<div id="container">
    <span id="p1">Paragraph 1</span>
    <span id="p2">Paragraph 2</span>
    This is just loose text.
    <p id="p3">Paragraph 3</p>
</div>

These are my definitions of strict and loose siblings:

I consider ID #p1 and #p2 strict siblings (because there is no text which is not encapsulated in any kind of html element between them.
And ID #p2 and #p3 to be loose siblings.

Given any element with a next sibling is it possible to know if the next sibling is strict or loose?

Edit: The siblings may be of different type and I updated the fiddle to reflect that.

Solution

Answering your question of

Given any element with a next sibling is it possible to know if the next sibling is strict or loose?

rather than following your lead from the fiddle, yes it's possible.

function isMyNextSiblingStrict(element)
{
    return ("nodeType" in element && element.nodeType === 1 && element.nextSibling !== null && element.nextSibling.nodeType === 1);
}

This will return true when an element's next sibling is another element. However, be careful with your example as it is incorrect by your own definition, the two spans have a text node between them made up of white space, so #1 and #2 are not "strict siblings".

<div id="container">
    <span id="p1">Paragraph 1</span><!-- WHITESPACE
 --><span id="p2">Paragraph 2</span>
    This is just loose text.
    <p id="p3">Paragraph 3</p>
</div>

These would be "strict siblings".

<div id="container">
    <span id="p1">Paragraph 1</span><span id="p2">Paragraph 2</span>
    This is just loose text.
    <p id="p3">Paragraph 3</p>
</div>

edit - just for fun I've created a jQuery extension that should do what you want - you can see it in your updated jsFiddle, I've not tested it much but it seems to work as you describe

$.fn.extend({
    siblingsStrict: function( until, selector ) {
        var ret = jQuery.map( this, function( elem ) {
            var n = ( elem.parentNode || {} ).firstChild,
                r = [],
                contig = false;

        for ( ; n; n = n.nextSibling ) {
                if ( n === elem ) {
                    contig = true;
                }
                else if ( n.nodeType === 1 && n !== elem ) {
                    r.push( n );
                }
                else if ( n.nodeType === 3 && n.textContent.replace(/\s/g, "") === "" ) {
                    continue;
                }
                else if ( n.nodeType === 3 && contig === true ) {
                    break;
                }
                else if ( n.nodeType === 3 && contig === false) {
                    r = [];
                }
        }

        return r;
    }, until );

        if ( name.slice( -5 ) !== "Until" ) {
            selector = until;
        }

        if ( selector && typeof selector === "string" ) {
            ret = jQuery.filter( selector, ret );
        }

        if ( this.length > 1 ) {
            // Remove duplicates
            if ( !guaranteedUnique[ name ] ) {
                ret = jQuery.unique( ret );
            }

            // Reverse order for parents* and prev-derivatives
            if ( rparentsprev.test( name ) ) {
                ret = ret.reverse();
            }
        }

        return this.pushStack( ret );
    }
});