Is it possible to have a pseudo element within pseudo element?

Question

div { position: relative; width: 100px; height: 100px; background: #f00; }
div::before { position: absolute; content: ''; width: 75px; height: 75px; background: #0f0; }
div::before::before { position: absolute; content: ''; width: 50px; height: 50px; background: #00f; }

Is my syntax wrong or is pseudo-element within pseudo-element not supported?

Note that I am aware about the ::after pseudo-element, though I need an actual element within another pseudo-element to achieve, e.g. where ::after does not suffice is:

div { position: relative; width: 100px; height: 100px; background: #f00; }
div::before { position: absolute; content: ''; right: 0; bottom: 0; width: 75px; height: 75px; background: #0f0; }
div::after { position: absolute; content: ''; left: 0; top: 0; width: 50px; height: 50px; background: #00f; }

Because ::after is relative to the element and not ::before.

Answer 1

Is my syntax wrong or is pseudo-element within pseudo-element not supported?

If I understand you correctly, it isn't possible. You can't chain multiple pseudo-elements as of Selectors Level 3 though it apparently may be allowed in the future.

Only one pseudo-element may appear per selector, and if present it must appear after the sequence of simple selectors that represents the subjects of the selector. Note: A future version of this specification may allow multiple pseudo-elements per selector.

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Interestingly, you can chain the ::first-letter & ::before / ::after pseudo-elements with the placeholder pseudo element, e.g.

::-webkit-input-placeholder::first-letter {
color: purple;  
}

http://jsfiddle.net/k3yb6/1/