Idnits ToolCheck internet-drafts for submission nits
idnits looks for violations of Section 2.1 and 2.2 of the requirements listed on http://www.ietf.org/ID-Checklist.html, for violations of http://www.ietf.org/ietf/1id-guidelines.txt, for missing or incorrect use of required boilerplate determined by the IETF Trust Legal Provisions https://trustee.ietf.org/license-info, and for various other nits such as unused, un-defined, and obsolete references. There are also a series of other nits identified.
idnits reports results in four levels, where errors are clear violations of IETF Trust boilerplate requirements, 1id-guidelines violations, and IESG guidelines violations. The lower levels (flaws, warnings, and comments) are advisory, and may not need changes to the document at all. They are intended to provide help in producing good documents, not to be a straitjacket. Common sense is expected when dealing with those.
As an example, idnits will warn about multiple spaces inside running text, when they are not preceded by punctuation. This is done in order to help avoid erroneous spacing; but in some cases such spacing may be deliberate, and part of the document formatting. If so, common sense and the document author's intentions should override the heuristics of the tool.