Query words in "the headings or body text" are Google's "most basic signal" of relevance, yet a searcher for "dogs" doesn't "want a page with the word 'dogs' on it hundreds of times". Keyword stuffing and hidden text (white on white, off-screen CSS) violate the spam policies. Keyword-density targets, the "first 100 words" rule, TF-IDF formulas and "LSI keywords" are unconfirmed: Google's documentation never mentions them. It documents RankBrain and neural matching, which relate words to concepts (Ranking Systems), so write the words and synonyms your readers use.