The three forms
Named: €. Decimal numeric: €. Hex numeric: €. All three render . Every character page on this site lists all three in its code section.
The 4 you must escape
& (&), < (<), > (>), " (") inside attributes. A raw & before text that looks like an entity name silently corrupts pages (“fish & chips” is fine; “AT&T” can parse oddly — always escape).
The 8 worth memorizing
© ©, ® ®, ™ ™, ° °, — —, – –, … …, (non-breaking space — the only one with no UTF-8 literal alternative that behaves the same).
Modern rule: declare UTF-8, write literals
With <meta charset="utf-8">, typing é directly is fully valid HTML5 — entities for visible characters are a legacy habit from Latin-1 days. Keep literals in source (readable, greppable), keep entities for the 4 syntax characters and for generated content where encoding is uncertain (emails, embeds). See Unicode explained.