Platform subtitle delivery lives or dies on reading speed and line discipline. The Netflix Timed Text Style Guide allows up to 20 characters per second on adult programs and 17 CPS on children’s programs, 42 characters per line, a maximum of two lines per subtitle event, a minimum duration of 5/6 of a second (20 frames at 24 fps) and a maximum of 7 seconds per event, and requires at least a 2-frame gap between events at any frame rate (their timing guide allows chaining gaps up to exactly 2 frames). The BBC Subtitle Guidelines keep the teletext heritage of 37 fixed-width characters per line and two lines, recommend a reading speed of 160-180 words per minute (about 0.3 seconds of on-screen time per word as a floor) and ask for gaps of a second and a half where speech pauses. YouTube accepts SRT and WebVTT as plain UTF-8 text: style markup like <i>/<b>/<u>/<font> is simply not recognized in SRT and positioning/styling in VTT is limited, so markup is reported as a warning rather than silently lost. This checker parses SRT, WebVTT, SBV and TTML with the shared subtitle engine, measures every cue (characters per line including spaces, lines per cue, CPS = characters ÷ duration, words per minute, durations and inter-cue gaps converted to frames at the frame rate you declare) and grades each violation as fail (hard spec limit) or warn (guideline recommendation), with overlap, negative-duration, empty-cue and non-monotonic structural checks always on. The report card summarises the tightest offenders and lists every finding by cue number, so a file can be fixed against the exact rule before delivery.