1.2em is generally the default line-height used by all css UAs, and it's also regarded as the optimal leading for readability, so there's little reason to vary it except in certain special circumstances. InDesign will apply the leading specified in the paragraph style, but if you leave that on Auto (i.e. 12pt text will have a leading listed as (14.4pt) ) then it will produce a line-height of 1.2 and you won't run into problems with line collisions.
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