Max length of an HTML attribute value

Question

How would I know what the maximum length of an element's attribute value is?

e.g.

<div id="value1..."></div>

What's the maximum allowable length for ID attribute of this div and any other attributes of any elements?

Answer 1

According to the SGML Declaration of HTML 4, the limit could be 65536 characters:

ATTSPLEN 65536   -- These are the largest values --
LITLEN   65536   -- permitted in the declaration --
NAMELEN  65536   -- Avoid fixed limits in actual --
PILEN    65536   -- implementations of HTML UA's --

The LITLEN limits the number of characters which can appear in a single attribute value literal and the ATTSPLEN limits the sum of all lengths of all attribute value specifications which appear in a tag.

Apparently, the limits used to be much lower in HTML 2, and people were complaining back then in 1995.

Answer 2

According to this page, technically unlimited. An actual test in various browsers will show their limits but my guess is the limit would be unreasonably long.