On-demand rendering

<section class="stage"><div class="heading">ON DEMAND <span class="hint">first visit → cached</span></div><div class="demand"><div class="people"><span>visitor 1</span><span>visitor 2</span></div><div class="arrow">→</div><div class="tile generator">RENDER<div class="gear">+</div></div><div class="arrow">→</div><div class="tile cache">/item/42.html</div></div><div class="caption">Only the first visit runs the renderer</div></section>

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