...and if the target attribute is invalid in a particular doctype, it doesn't matter whether you set it declaratively (in the document source) or programmatically, except that there wouldn't be a validation error on load. Instead, the UA should either return an "invalid attiribute" error when the code runs or ignore the nonsense attribute once it is set. (Which of those should happen depends on whether you believe users should report concrete errors or "weird behaviour" when they file their trouble tickets.)
...and if the target attribute is invalid in a particular doctype, it doesn't matter whether you set it declaratively (in the document source) or programmatically, except that there wouldn't be a validation error on load. Instead, the UA should either return an "invalid attiribute" error when the code runs or ignore the nonsense attribute once it is set. (Which of those should happen depends on whether you believe users should report concrete errors or "weird behaviour" when they file their trouble tickets.)