How should I escape attributes in the css/js attibute selector [attr=value]?
Specifically, is this correct?
document.querySelector('input[name="test[33]"]')
I'm looking for the "standard way" of doing this, if any, because I don't want Sizzle using a heavy-to-execute fallback function
Yes, that is one correct approach. The Selectors Level 3 specification states the following:
Attribute values must be CSS identifiers or strings.
The example in your question uses a string as the attribute value. An "identifier" is defined as follows:
In CSS, identifiers... can contain only the characters [a-zA-Z0-9] and ISO 10646 characters U+00A0 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, two hyphens, or a hyphen followed by a digit. Identifiers can also contain escaped characters and any ISO 10646 character as a numeric code...
So following that, it is also legal to escape the special characters and omit the quotes:
document.querySelector('input[name=test\\[33\\]]')