I just installed FFmpeg/FFDshow codec to try if they could work H264 with Opera

Opera can't use them as far as I know.

Do you mean I will unintall them ?

No, I mean that Opera can only play H264 (and some other codecs) if Windows can play them natively. In other words, Windows should be able to play those codecs just after having been installed.

Problems Detected

Did you check the whole page? Real problems are usually listed on the bottom of the page?

Also, are the items under 'Graphics Feature Status' with 'hardware accelerated' status? At least most of them.

GPU was loaded successfully,I have seen the GPU process

"Problems Detected
Some drivers are unable to reset the D3D device in the GPU process sandbox
Applied Workarounds: exit_on_context_lost
TexSubImage is faster for full uploads on ANGLE
Applied Workarounds: texsubimage_faster_than_teximage
Clear uniforms before first program use on all platforms: 124764, 349137
Applied Workarounds: clear_uniforms_before_first_program_use
Always rewrite vec/mat constructors to be consistent: 398694
Applied Workarounds: scalarize_vec_and_mat_constructor_args
ANGLE crash on glReadPixels from incomplete cube map texture: 518889
Applied Workarounds: force_cube_complete
On Intel GPUs MSAA performance is not acceptable for GPU rasterization: 527565
Applied Workarounds: disable_msaa_on_non_webgl_contexts
Framebuffer discarding can hurt performance on non-tilers: 570897
Applied Workarounds: disable_discard_framebuffer
Direct composition flashes black initially on Win <10: 588588
Applied Workarounds: disable_direct_composition
Accelerated rasterization has been disabled, either via about:flags or command line.
Disabled Features: rasterization
Native GpuMemoryBuffers have been disabled, either via about:flags or command line.
Disabled Features: native_gpu_memory_buffers"

My WMP can play MP4 which encoded in H264

And you never installed any codec?

my graphic card is intel HD Graphics 4600 which does not support HW acceleration

Yes, it does support.

it display "initialization failed"

Does opera://gpu shows any error message? Do you see a GPU process in Opera's task manager?

"render_id: 172
player_id: 1
pipeline_state: kStopped
event: WEBMEDIAPLAYER_DESTROYED
url: http://h264.code-shop.com/demo/apache/workers_world_co64_box64.mp4?start=0&end=60
total_bytes: 50265908
streaming: false
single_origin: true
passed_cors_access_check: false
range_header_supported: true
pipeline_error: pipeline: initialization failed"

it display "initialization failed"

Opera need system natively decode support,does it mean HW decode accelerate？or mean software decode on OS?

It means that Windows Media Player (or whatever media player that comes with Windows) should be able to play the file encoded in h264 without needing any codec pack to be installed.

OS is win 7

Win7 should be able to play h264 videos.

It's 32 or 64 bits? Your graphic cards supports hardware acceleration?

Try taking a look at opera://media-internals/ while playing a video (or trying to play), it may bring some info to you.

@leocg ,it does not play,for example ,also can't playback twitter video.
I notice @avl ever said,on Windows platform,Opera need system natively decode support,does it mean HW decode accelerate？or mean software decode on OS?