font element

<font color="red" face="Arial" size="5"> set the colour, typeface and size of a run of text before stylesheets existed. It is the emblem of 1990s markup, and it lives on in email templates, WYSIWYG editor output and content pasted in from word processors, because every one of those tools could emit it.

Why avoid

It is obsolete and non-conforming. The HTML Standard lists font among the elements that “are entirely obsolete, and must not be used by authors”, with basefont, big, center and tt, and the instruction “Use appropriate elements or CSS instead.”

It still works, which is how it spreads. The Standard’s rendering rules keep all three attributes alive as presentational hints: color sets the color property, face sets font-family, and size sets font-size through “the rules for parsing a legacy font size”, the old 1-to-7 scale with relative +2 and -1 steps. The page’s typography ends up spread across the content, one element at a time. A stylesheet can’t restyle it in one place, and a redesign turns into an edit of every document that contains it.

It is also forgiving in a way that hides mistakes. Its color attribute is read with “the rules for parsing a legacy color value”, which “replace any character in input that is not an ASCII hex digit with U+0030 (0)” rather than rejecting the value. A misspelled colour name doesn’t fail. It turns into some other colour, where CSS would have discarded the invalid declaration.

And it means nothing. <font color="red">Payment failed</font> looks urgent, but the element carries no semantics, so to a screen reader it’s just more text.

Use instead

Put the style in CSS, and the meaning in an element that has one:

.notice {
  color: #b00020;
  font-family: Georgia, serif;
  font-size: 1.25rem;
}

<p class="notice"><strong>Payment failed.</strong> Please try again.</p>

Each attribute has a direct CSS counterpart: color becomes color, face becomes font-family, and size becomes font-size.

Detectability

Fully detectable by tag name. There is no autofix: font still applies its colour, typeface and size, so removing it would visibly change the text.

Resources

• HTML Standard — Non-conforming features — font is entirely obsolete: “Use appropriate elements or CSS instead.”
• HTML Standard — Rendering: phrasing content — color, face and size on font are presentational hints for color, font-family and font-size.
• HTML Standard — Parsing a legacy color value — non-hex characters are replaced with 0 instead of being rejected.
• MDN — <font> — deprecated; its attributes and their CSS replacements.