I have read ffprobe document at here.
I don't know why my first time of the frame is 11.745 seconds, even given "-read_intervals 12%13". (12sec ~ 13sec )
$ ffprobe -i chrome.webm -show_frames -select_streams v:0 -read_intervals 12%13 -hide_banner -loglevel panic
[FRAME]
media_type=video
stream_index=0
key_frame=1
pkt_pts=11745
pkt_pts_time=11.745000
pkt_dts=11745
pkt_dts_time=11.745000
best_effort_timestamp=11745
best_effort_timestamp_time=11.745000
pkt_duration=33
pkt_duration_time=0.033000
pkt_pos=717643
pkt_size=5864
width=480
height=270
pix_fmt=yuv420p
sample_aspect_ratio=1:1
pict_type=I
coded_picture_number=0
display_picture_number=0
interlaced_frame=0
top_field_first=0
repeat_pict=0
[/FRAME]
...