Rather obvious, but nevertheless…
Using Intel iGPU + Intel VAAPI driver.
Freshly installed OS: H.264 (AVC) doesn’t work - as expected.
After installing Intel VAAPI driver: can play some videos, but with extra CPU load, cannot play some others (depends on sub-formats?).
Installing codecs from VLC repo solved this issue.
Some months ago user complains about Ivy Bridge CPU which cannot play videos - maybe that case.