The warning grammar

shape is syntax, pictogram is content

Unicode: U+26A0 warning sign · the standard’s own signs are not characters
The job: Tell a person, before they can read anything, what kind of message this is — and only then what it concerns.
Standards: The most complete published grammar of any symbol set on this site. ISO 3864 fixes the safety colours and geometric shapes; ISO 7010 registers the individual signs against them. The syntax is carried entirely by shape and colour, independent of the picture inside: yellow triangle warns, blue circle commands, red circle with a diagonal bar prohibits, green square denotes safety and escape, red square denotes firefighting equipment. Learn five shapes and every registered sign parses — you know the speech act before you identify the hazard.
Failure modes: The pictogram inside the frame is far less legible than the frame around it, and many registered signs test poorly for comprehension — which is why ISO 9186 exists as a procedure for measuring whether a graphical symbol is understood at all. The redundancy of shape plus colour is deliberate insurance against colour-vision deficiency, and it is defeated the moment a designer keeps the colour and drops the shape. Separately: ⚠ the Unicode character has no standing in ISO 7010 — the generic exclamation triangle in software borrows the authority of the standard without meeting it.
Cultural flips: The standard works by overriding local colour meaning rather than accommodating it. Red reads as danger here and as luck and celebration in Chinese usage; white is mourning across much of East Asia. ISO does not negotiate with any of that — it teaches a grammar and expects it to be learned, which is a different theory of how symbols work from anything else in this institute.

Sources · 4

documented ISO 3864-1 — safety colours and geometric shapes; the layer that carries the syntax.
documented ISO 7010 — the registered safety signs, catalogued by prefix: W warning, M mandatory, P prohibition, E escape, F fire.
documented ISO 9186 — methods for testing the comprehensibility of graphical symbols. A standard for finding out whether a standard worked.
documented The Unicode Standard, U+26A0 WARNING SIGN — a character, not a registered safety sign.