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CSS tests use regular expressions matchers that do return the start position of each match. It should be relatively straightforward to compute the line/column position from there, to be able to return more precise messages for MEASURES and STYLE_SHEETS_USE.
Short correction: the bug applies to MEASURES and STYLE_SHEETS_SUPPORT tests (and not STYLE_SHEETS_USE)
The character position gets converted to a line position in AbstractCSSTestImplementation and added to the error message in MeasuresTest and StyleSheetsSupportTest. Column position not set for the time being. It could be done easily in the future though. Related test cases in the test suite: MEASURES 1 MEASURES 2 MEASURES 3 MEASURES 4 MEASURES 5 MEASURES 6 MEASURES 7 STYLE_SHEETS_SUPPORT 1 STYLE_SHEETS_SUPPORT 2 STYLE_SHEETS_SUPPORT 3 STYLE_SHEETS_SUPPORT 5