Proper Keyword Density Range for each search engine?

As we know,when the Keyword Desity is too low,search engines may think lowly of the Keyword.
When the Keyword Desity is too high,serach engines may penalize the corresponding website.
And different serach engines have different analysis algorithms for the Keyword Desity.
Then,here comes the question--What is the proper Keyword Desity Range for each search engine?
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Search Engine Optimization|Digital Agency