For dc:title (https://www.w3.org/TR/2021/WD-epub-33-20210224/#sec-opf-dctitle), it is only at the very end of this section that one finds "This specification imposes no additional restrictions or requirements on the title except that it MUST be at least one character in length after white space has been trimmed.."

For DCMES optional elements (https://www.w3.org/TR/2021/WD-epub-33-20210224/#sec-opf-dcmes-optional), the text is just after the formal definition of the elements (the blue box) and slightly different: "The value of all OPTIONAL [DC11] elements MUST be at least one character in length after white space has been trimmed."

Suggestion 1 is to adopt the position and wording of "DCMES optional elements" for "dc:title".

Suggestion 2: I think the intent is that consumers of those elements should trim the white space. Strictly speaking, that is not implied by the text. May be change it to: "Initial and final white space in the value of {this element/all OPTIONAL [DC11] elements} is not significant and there MUST be at least one character in length after white space has been trimmed."

Also: do you intend only trimming of initial and final whitespace, or something more like CSS (e.g. collapsing multiple spaces in the middle)?