apple-touch-icon-precomposed Explained
apple-touch-icon-precomposed is a legacy rel value that told early iOS not to apply its glossy "shine" to a home screen icon. The image was used exactly as composed. Since iOS 7 dropped that gloss effect, the precomposed variant no longer changes anything, and a plain apple-touch-icon is all you need.
What "precomposed" meant
Up to iOS 6, Safari added a glossy highlight and rounded corners to any home screen icon automatically. Declaring the icon as precomposed opted out of the shine, so a design that already had its own finish wasn't given a second one on top.
iOS 7 removed the automatic gloss entirely, which made the distinction moot. Both rel values now produce the same home screen icon, so the precomposed variant is still honoured but has nothing left to do.
The two variants
<!-- legacy: opted out of the old iOS gloss -->
<link rel="apple-touch-icon-precomposed" href="/apple-touch-icon.png" />
<!-- today: just use this -->
<link rel="apple-touch-icon" href="/apple-touch-icon.png" />Precomposed icon FAQ
Do I need apple-touch-icon-precomposed?
No. Since iOS 7 there's no gloss to opt out of, so a plain apple-touch-icon behaves identically. Ship just that.
What's the difference between apple-touch-icon and the precomposed version?
Historically, precomposed skipped iOS's automatic glossy shine. On current iOS neither adds a shine, so there is no practical difference anymore.
Does precomposed stop the rounded corners?
No. iOS still masks home screen icons to its own corner radius regardless. Precomposed only ever affected the gloss overlay, which iOS no longer applies.