input[type='text'] CSS selector does not apply to default-type text inputs?

The CSS uses only the data in the DOM tree, which has little to do with how the renderer decides what to do with elements with missing attributes.

So either let the CSS reflect the HTML

input:not([type]), input[type="text"]
{
background:red;
}

or make the HTML explicit.

<input name='t1' type='text'/> /* Is Not Red */

If it didn't do that, you'd never be able to distinguish between

element { ...properties... }

and

element[attr] { ...properties... }

because all attributes would always be defined on all elements. (For example, table always has a border attribute, with 0 for a default.)

By CSS specifications, browsers may or may not use information about default attributes; mostly the don’t. The relevant clause in the CSS 2.1 spec is 5.8.2 Default attribute values in DTDs. In CSS 3 Selectors, it’s clause 6.3.4, with the same name. It recommends: “Selectors should be designed so that they work whether or not the default values are included in the document tree.”

It is generally best to explicitly specify essential attributes such as type=text instead of defaulting them. The reason is that there is no simple reliable way to refer to the input elements with defaulted type attribute.

Because, it is not supposed to do that.

input[type=text] { } is an attribute selector, and will only select those element, with the matching attribute.