Detailed Answer
Most are reasonably accurate for measuring gross WPM and accuracy if you take them honestly (no backspacing abuse, no retaking until perfect). However, accuracy varies: sites that count only complete words (ignoring single-letter errors) inflate scores. The most accurate tests are those that count every character and deduct errors from net WPM (e.g., TypingTest.com, Monkeytype with 'strict' mode). For official certification, use proctored exams. Free tests are fine for self-assessment but not for job proof.