If an HTML document has no associated doctype (e.g. <!DOCTYPE html>), it's in quirks mode.

Verify that by checking doctype or compatMode:

document.doctype === null;
document.compatMode === 'BackCompat';

Note: <meta http-equiv="X-UA-Compatible" content="IE=Edge"> won't change whether a document is in quirks mode.

For documents in quirks mode on IE/Edge, a table cell element (<th> or <td>) that has width: auto set explicitly will always have its white-space computed to normal regardless of its set value even that has !important specified.

We say "has width: auto set explicitly" here because width is auto by default, while the quirk happens only when it's set explicitly.

Compare the following example on IE/Edge and another browser:

Source code of the example available here.

So the conclusion is that unless you want certain quirks mode behavior (not a good idea though), always include a valid doctype to put the document into no-quirks mode (standards mode) so that unexpected behavior won't get in the way.