No space between the doctype public and system identifiers.

The doctype's public identifier and system identifier are written back to back, with no space between their two quoted strings.

Legacy doctypes from HTML 4.01 and XHTML 1.0 have two quoted strings after the PUBLIC keyword. The first is the public identifier, which names the DTD, such as "-//W3C//DTD XHTML 1.0 Transitional//EN". The second is the system identifier, the URL of that DTD. The HTML syntax requires whitespace between them, so a closing quote followed directly by an opening quote (//EN""http://) is a parse error. The same applies to single-quoted identifiers.

These doctypes are usually split across two lines, with the URL on the second line. A minifier or template that removes the line break without leaving a space produces this error.

Browsers recover from it: they still read both identifiers, so the page renders in the same mode as it would with the space. The legacy doctype itself is still reported, as "Almost standards mode doctype" or "Obsolete doctype". Adding a space clears this error only. Replacing the declaration with <!DOCTYPE html>, which has no identifiers, clears both.

Invalid example

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN""http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
  <head>
    <title>Products</title>
  </head>
  <body>
    <p>Hello</p>
  </body>
</html>

Valid example

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Products</title>
  </head>
  <body>
    <p>Hello</p>
  </body>
</html>

Learn more:

MDN Reference

MDN Web Docs: Doctype

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MDN Reference

MDN Web Docs: Quirks Mode and Standards Mode

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WHATWG Spec

HTML Living Standard: The HTML syntax

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