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"For example, the markup <input type="range" min=0 max=100 step=20 value=50> results in a range control whose initial value is 60."
Actually, the spec is correct: [[ When the element is suffering from a step mismatch, the user agent must round the element's value to the nearest number for which the element would not suffer from a step mismatch, and which is greater than or equal to the minimum, and, if the maximum is not less than the minimum, which is less than or equal to the maximum, if there is a number that matches these constraints. If two numbers match these constraints, then user agents must use the one nearest to positive infinity. For example, the markup <input type="range" min=0 max=100 step=20 value=50> results in a range control whose initial value is 60. ]] 50 is a step mismatch. It could be rounded to 40 and 60, and 60 is the one nearest to positive infinity.