text-indent changes the indentation or offset of the first line, generally in a paragraph. If it is a percent then it is a percentage of the containing block (p contains the text) so if the page is resized or viewed on a different monitor and the paragraph changes width, the indentation will change. ___pt is an absolute value and won't change like the % will. See w3schools for more. Some of the values, e.g. a negative amount, have interesting effects.
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