UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.117 Safari/537.36

Example URL:
https://jsfiddle.net/4m7spd4e/

Steps to reproduce the problem:
1. Do heavy-stressing operation on your computer (stress CPU and I/O)
2. Open provided URL (https://jsfiddle.net/4m7spd4e/)
3. Wait for video to start playing
4. Observe logs
5. Observe TC results (green = pass, red = fail)

What is the expected behavior?
Simply:
Interval between two `timeupdate` fired events should be smaller then 250ms (spec says it should be in range [15ms, 250ms]).

From HTML5 specification (https://dev.w3.org/html5/spec-preview/media-elements.html):

"If the time was reached through the usual monotonic increase of the current playback position during normal playback, and if the user agent has not fired a timeupdate event at the element in the past 15 to 250ms and is not still running event handlers for such an event, then the user agent must queue a task to fire a simple event named timeupdate at the element. (In the other cases, such as explicit seeks, relevant events get fired as part of the overall process of changing the current playback position.)"

"The event thus is not to be fired faster than about 66Hz or slower than 4Hz (assuming the event handlers don't take longer than 250ms to run). User agents are encouraged to vary the frequency of the event based on the system load and the average cost of processing the event each time, so that the UI updates are not any more frequent than the user agent can comfortably handle while decoding the video."

What went wrong?
Chromium/Chrome sometimes fires timeupdate too later, after more then 250ms already passed then last timeupdate event. This happens due to scheduling of internal tasks in BLINK.

The problem is that in HTMLMediaElement::ScheduleTimeupdateEvent() we schedule next timeupdate event after (at lest! which is crucial here!) 250 ms (value of constant kMaxTimeupdateEventFrequency).

Let's consider now the scenario, where:
1. At 0ms time position we trigger `timeupdate` event and schedule next one at 250ms time position
2. At 240ms time position a task X is taken from task runner's queue and execution of it is started
3. Task X finishes at 300ms time position (or later), since task runners cannot interrupt running tasks, we had to wait for task X to finish.
4. Delayed post-task which schedules `timeupdate` event is executed now and triggers `timeupdate` event at 300ms time position (or later).

This clearly shows, that on slower devices/platforms `timeupdate` event can be fired much later then it should be.

Did this work before? No 

Is it a problem with Flash or HTML5? HTML5

Does this work in other browsers? N/A

Chrome version: 66.0.3359.117  Channel: stable
OS Version: Ubuntu 16.04
Flash Version: 

Contents of chrome://gpu: 

Graphics Feature Status
Canvas: Hardware accelerated
CheckerImaging: Disabled
Flash: Hardware accelerated
Flash Stage3D: Hardware accelerated
Flash Stage3D Baseline profile: Hardware accelerated
Compositing: Hardware accelerated
Multiple Raster Threads: Enabled
Native GpuMemoryBuffers: Software only. Hardware acceleration disabled
Rasterization: Software only. Hardware acceleration disabled
Surface Synchronization: Enabled
Video Decode: Unavailable
WebGL: Hardware accelerated
WebGL2: Hardware accelerated
Driver Bug Workarounds
adjust_src_dst_region_for_blitframebuffer
clear_uniforms_before_first_program_use
count_all_in_varyings_packing
decode_encode_srgb_for_generatemipmap
disable_framebuffer_cmaa
disable_post_sub_buffers_for_onscreen_surfaces
disable_software_to_accelerated_canvas_upgrade
dont_remove_invariant_for_fragment_input
force_cube_map_positive_x_allocation
force_int_or_srgb_cube_texture_complete
init_texture_max_anisotropy
regenerate_struct_names
remove_invariant_and_centroid_for_essl3
scalarize_vec_and_mat_constructor_args
Problems Detected
Accelerated video decode is unavailable on Linux: 137247
Disabled Features: accelerated_video_decode
Clear uniforms before first program use on all platforms: 124764, 349137
Applied Workarounds: clear_uniforms_before_first_program_use
Mesa drivers in Linux handle varyings without static use incorrectly: 333885
Applied Workarounds: count_all_in_varyings_packing
Linux AMD drivers incorrectly return initial value of 1 for TEXTURE_MAX_ANISOTROPY: 348237
Applied Workarounds: init_texture_max_anisotropy
Always rewrite vec/mat constructors to be consistent: 398694
Applied Workarounds: scalarize_vec_and_mat_constructor_args
Linux AMD drivers handle struct scopes incorrectly: 403957
Applied Workarounds: regenerate_struct_names
Linux ATI drivers crash on binding incomplete cube map texture to FBO: 518889
Applied Workarounds: force_cube_map_positive_x_allocation
Use GL_INTEL_framebuffer_CMAA on ChromeOS: 535198
Applied Workarounds: disable_framebuffer_cmaa
Disable partial swaps on Mesa drivers (detected with GL_VERSION): 339493
Applied Workarounds: disable_post_sub_buffers_for_onscreen_surfaces
Decode and encode before generateMipmap for srgb format textures on os except macosx: 634519
Applied Workarounds: decode_encode_srgb_for_generatemipmap
adjust src/dst region if blitting pixels outside read framebuffer on Linux AMD: 664740
Applied Workarounds: adjust_src_dst_region_for_blitframebuffer
AMD drivers in Linux require invariant qualifier to match between vertex and fragment shaders: 659326, 639760
Applied Workarounds: remove_invariant_and_centroid_for_essl3, dont_remove_invariant_for_fragment_input
Disable KHR_blend_equation_advanced until cc shaders are updated: 661715
Applied Workarounds: disable(GL_KHR_blend_equation_advanced), disable(GL_KHR_blend_equation_advanced_coherent)
Decode and Encode before generateMipmap for srgb format textures on Linux AMD: 634519
Applied Workarounds: decode_encode_srgb_for_generatemipmap
Software to Accelerated canvas update breaks Linux AMD: 710029
Applied Workarounds: disable_software_to_accelerated_canvas_upgrade
Force integer or srgb cube map texture complete on Linux AMD: 712117
Applied Workarounds: force_int_or_srgb_cube_texture_complete
Don't expose disjoint_timer_query extensions to WebGL: 808744
Native GpuMemoryBuffers have been disabled, either via about:flags or command line.
Disabled Features: native_gpu_memory_buffers
Checker-imaging has been disabled via finch trial or the command line.
Disabled Features: checker_imaging
Version Information
Data exported	2018-04-26T11:58:43.463Z
Chrome version	Chrome/66.0.3359.117
Operating system	Linux 4.13.0-39-generic
Software rendering list URL	https://chromium.googlesource.com/chromium/src/+/7f59c28e25283df88e0c6ac8d8b2551d8c6ad93b/gpu/config/software_rendering_list.json
Driver bug list URL	https://chromium.googlesource.com/chromium/src/+/7f59c28e25283df88e0c6ac8d8b2551d8c6ad93b/gpu/config/gpu_driver_bug_list.json
ANGLE commit id	22c768fbda54
2D graphics backend	Skia/66 773868fdade5f9f0e7697e6d09c9bd80aaa9b402-
Command Line	/usr/bin/google-chrome-stable --flag-switches-begin --flag-switches-end
Driver Information
Initialization time	39
In-process GPU	false
Passthrough Command Decoder	false
Direct Composition	false
Supports overlays	false
Sandboxed	false
GPU0	VENDOR = 0x1002, DEVICE= 0x6610 *ACTIVE*
Optimus	false
Optimus	false
AMD switchable	false
Driver vendor	Mesa
Driver version	17.2.8
Driver date	
Pixel shader version	1.30
Vertex shader version	1.30
Max. MSAA samples	8
Machine model name	
Machine model version	
GL_VENDOR	X.Org
GL_RENDERER	AMD OLAND (DRM 2.50.0 / 4.13.0-39-generic, LLVM 5.0.0)
GL_VERSION	3.0 Mesa 17.2.8
GL_EXTENSIONS	GL_AMD_conservative_depth GL_AMD_draw_buffers_blend GL_AMD_performance_monitor GL_AMD_pinned_memory GL_AMD_seamless_cubemap_per_texture GL_AMD_shader_stencil_export GL_AMD_shader_trinary_minmax GL_ANGLE_texture_compression_dxt3 GL_ANGLE_texture_compression_dxt5 GL_APPLE_packed_pixels GL_ARB_ES2_compatibility GL_ARB_ES3_compatibility GL_ARB_arrays_of_arrays GL_ARB_base_instance GL_ARB_bindless_texture GL_ARB_blend_func_extended GL_ARB_buffer_storage GL_ARB_clear_buffer_object GL_ARB_clear_texture GL_ARB_clip_control GL_ARB_color_buffer_float GL_ARB_compressed_texture_pixel_storage GL_ARB_compute_shader GL_ARB_compute_variable_group_size GL_ARB_conditional_render_inverted GL_ARB_conservative_depth GL_ARB_copy_buffer GL_ARB_copy_image GL_ARB_cull_distance GL_ARB_debug_output GL_ARB_depth_buffer_float GL_ARB_depth_clamp GL_ARB_depth_texture GL_ARB_derivative_control GL_ARB_draw_buffers GL_ARB_draw_buffers_blend GL_ARB_draw_elements_base_vertex GL_ARB_draw_instanced GL_ARB_explicit_attrib_location GL_ARB_explicit_uniform_location GL_ARB_fragment_coord_conventions GL_ARB_fragment_program GL_ARB_fragment_program_shadow GL_ARB_fragment_shader GL_ARB_framebuffer_no_attachments GL_ARB_framebuffer_object GL_ARB_framebuffer_sRGB GL_ARB_get_program_binary GL_ARB_get_texture_sub_image GL_ARB_half_float_pixel GL_ARB_half_float_vertex GL_ARB_instanced_arrays GL_ARB_internalformat_query GL_ARB_internalformat_query2 GL_ARB_invalidate_subdata GL_ARB_map_buffer_alignment GL_ARB_map_buffer_range GL_ARB_multi_bind GL_ARB_multisample GL_ARB_multitexture GL_ARB_occlusion_query GL_ARB_occlusion_query2 GL_ARB_pipeline_statistics_query GL_ARB_pixel_buffer_object GL_ARB_point_parameters GL_ARB_point_sprite GL_ARB_program_interface_query GL_ARB_provoking_vertex GL_ARB_query_buffer_object GL_ARB_robust_buffer_access_behavior GL_ARB_robustness GL_ARB_sample_shading GL_ARB_sampler_objects GL_ARB_seamless_cube_map GL_ARB_seamless_cubemap_per_texture GL_ARB_separate_shader_objects GL_ARB_shader_atomic_counter_ops GL_ARB_shader_atomic_counters GL_ARB_shader_ballot GL_ARB_shader_bit_encoding GL_ARB_shader_clock GL_ARB_shader_group_vote GL_ARB_shader_image_load_store GL_ARB_shader_image_size GL_ARB_shader_objects GL_ARB_shader_precision GL_ARB_shader_stencil_export GL_ARB_shader_storage_buffer_object GL_ARB_shader_texture_image_samples GL_ARB_shader_texture_lod GL_ARB_shading_language_100 GL_ARB_shading_language_420pack GL_ARB_shading_language_packing GL_ARB_shadow GL_ARB_stencil_texturing GL_ARB_sync GL_ARB_texture_barrier GL_ARB_texture_border_clamp GL_ARB_texture_compression GL_ARB_texture_compression_bptc GL_ARB_texture_compression_rgtc GL_ARB_texture_cube_map GL_ARB_texture_cube_map_array GL_ARB_texture_env_add GL_ARB_texture_env_combine GL_ARB_texture_env_crossbar GL_ARB_texture_env_dot3 GL_ARB_texture_float GL_ARB_texture_gather GL_ARB_texture_mirror_clamp_to_edge GL_ARB_texture_mirrored_repeat GL_ARB_texture_multisample GL_ARB_texture_non_power_of_two GL_ARB_texture_query_levels GL_ARB_texture_query_lod GL_ARB_texture_rectangle GL_ARB_texture_rg GL_ARB_texture_rgb10_a2ui GL_ARB_texture_stencil8 GL_ARB_texture_storage GL_ARB_texture_storage_multisample GL_ARB_texture_swizzle GL_ARB_texture_view GL_ARB_timer_query GL_ARB_transform_feedback2 GL_ARB_transform_feedback3 GL_ARB_transform_feedback_instanced GL_ARB_transpose_matrix GL_ARB_uniform_buffer_object GL_ARB_vertex_array_bgra GL_ARB_vertex_array_object GL_ARB_vertex_attrib_binding GL_ARB_vertex_buffer_object GL_ARB_vertex_program GL_ARB_vertex_shader GL_ARB_vertex_type_10f_11f_11f_rev GL_ARB_vertex_type_2_10_10_10_rev GL_ARB_window_pos GL_ATI_blend_equation_separate GL_ATI_draw_buffers GL_ATI_fragment_shader GL_ATI_meminfo GL_ATI_separate_stencil GL_ATI_texture_compression_3dc GL_ATI_texture_env_combine3 GL_ATI_texture_float GL_ATI_texture_mirror_once GL_EXT_abgr GL_EXT_bgra GL_EXT_blend_color GL_EXT_blend_equation_separate GL_EXT_blend_func_separate GL_EXT_blend_minmax GL_EXT_blend_subtract GL_EXT_compiled_vertex_array GL_EXT_copy_texture GL_EXT_depth_bounds_test GL_EXT_draw_buffers2 GL_EXT_draw_instanced GL_EXT_draw_range_elements GL_EXT_fog_coord GL_EXT_framebuffer_blit GL_EXT_framebuffer_multisample GL_EXT_framebuffer_multisample_blit_scaled GL_EXT_framebuffer_object GL_EXT_framebuffer_sRGB GL_EXT_gpu_program_parameters GL_EXT_multi_draw_arrays GL_EXT_packed_depth_stencil GL_EXT_packed_float GL_EXT_packed_pixels GL_EXT_pixel_buffer_object GL_EXT_point_parameters GL_EXT_polygon_offset GL_EXT_polygon_offset_clamp GL_EXT_provoking_vertex GL_EXT_rescale_normal GL_EXT_secondary_color GL_EXT_separate_specular_color GL_EXT_shader_integer_mix GL_EXT_shadow_funcs GL_EXT_stencil_two_side GL_EXT_stencil_wrap GL_EXT_subtexture GL_EXT_texture GL_EXT_texture3D GL_EXT_texture_array GL_EXT_texture_compression_dxt1 GL_EXT_texture_compression_latc GL_EXT_texture_compression_rgtc GL_EXT_texture_compression_s3tc GL_EXT_texture_cube_map GL_EXT_texture_edge_clamp GL_EXT_texture_env_add GL_EXT_texture_env_combine GL_EXT_texture_env_dot3 GL_EXT_texture_filter_anisotropic GL_EXT_texture_integer GL_EXT_texture_lod_bias GL_EXT_texture_mirror_clamp GL_EXT_texture_object GL_EXT_texture_rectangle GL_EXT_texture_sRGB GL_EXT_texture_sRGB_decode GL_EXT_texture_shared_exponent GL_EXT_texture_snorm GL_EXT_texture_swizzle GL_EXT_timer_query GL_EXT_transform_feedback GL_EXT_vertex_array GL_EXT_vertex_array_bgra GL_IBM_multimode_draw_arrays GL_IBM_rasterpos_clip GL_IBM_texture_mirrored_repeat GL_INGR_blend_func_separate GL_KHR_context_flush_control GL_KHR_debug GL_KHR_no_error GL_KHR_robust_buffer_access_behavior GL_KHR_robustness GL_MESA_pack_invert GL_MESA_shader_integer_functions GL_MESA_texture_signed_rgba GL_MESA_window_pos GL_NVX_gpu_memory_info GL_NV_blend_square GL_NV_conditional_render GL_NV_depth_clamp GL_NV_fog_distance GL_NV_light_max_exponent GL_NV_packed_depth_stencil GL_NV_primitive_restart GL_NV_texgen_reflection GL_NV_texture_barrier GL_NV_texture_env_combine4 GL_NV_texture_rectangle GL_NV_vdpau_interop GL_OES_EGL_image GL_OES_read_format GL_S3_s3tc GL_SGIS_generate_mipmap GL_SGIS_texture_border_clamp GL_SGIS_texture_edge_clamp GL_SGIS_texture_lod GL_SUN_multi_draw_arrays
Disabled Extensions	GL_KHR_blend_equation_advanced GL_KHR_blend_equation_advanced_coherent
Disabled WebGL Extensions	EXT_disjoint_timer_query EXT_disjoint_timer_query_webgl2
Window system binding vendor	SGI
Window system binding version	1.4
Window system binding extensions	GLX_ARB_create_context GLX_ARB_create_context_profile GLX_ARB_create_context_robustness GLX_ARB_fbconfig_float GLX_ARB_framebuffer_sRGB GLX_ARB_multisample GLX_EXT_create_context_es_profile GLX_EXT_create_context_es2_profile GLX_EXT_fbconfig_packed_float GLX_EXT_framebuffer_sRGB GLX_EXT_import_context GLX_EXT_libglvnd GLX_EXT_texture_from_pixmap GLX_EXT_visual_info GLX_EXT_visual_rating GLX_MESA_copy_sub_buffer GLX_OML_swap_method GLX_SGI_make_current_read GLX_SGI_swap_control GLX_SGIS_multisample GLX_SGIX_fbconfig GLX_SGIX_pbuffer GLX_SGIX_visual_select_group GLX_INTEL_swap_event
Window manager	Compiz
XDG_CURRENT_DESKTOP	Unity
GDMSESSION	ubuntu
Compositing manager	Yes
Direct rendering	Yes
Reset notification strategy	0x8261
GPU process crash count	0
System visual ID	33
RGBA visual ID	114
Compositor Information
Tile Update Mode	One-copy
Partial Raster	Enabled
GpuMemoryBuffers Status
ATC	Software only
ATCIA	Software only
DXT1	Software only
DXT5	Software only
ETC1	Software only
R_8	Software only
R_16	Software only
RG_88	Software only
BGR_565	Software only
RGBA_4444	Software only
RGBX_8888	Software only
RGBA_8888	Software only
BGRX_8888	Software only
BGRX_1010102	Software only
RGBX_1010102	Software only
BGRA_8888	Software only
RGBA_F16	Software only
YVU_420	Software only
YUV_420_BIPLANAR	Software only
UYVY_422	Software only
Display(s) Information
Info	Display[8564183066946816] bounds=0,0 1920x1080, workarea=57,24 1863x1056, scale=1, external
Color space information	{primaries:[[0.3590,0.4464,0.1589,],[0.1959,0.7428,0.0612,],[0.0097,0.0435,0.7717,],], transfer:0.0000*x + 0.0000 if x < 0.0000 else (1.0000*x + 0.0000)**2.2000 + 0.0000, matrix:RGB, range:FULL}
Bits per color component	8
Bits per pixel	24
Info	Display[8564183066946817] bounds=1920,0 1920x1080, workarea=1920,0 1920x1080, scale=1, external
Color space information	{primaries:[[0.3590,0.4464,0.1589,],[0.1959,0.7428,0.0612,],[0.0097,0.0435,0.7717,],], transfer:0.0000*x + 0.0000 if x < 0.0000 else (1.0000*x + 0.0000)**2.2000 + 0.0000, matrix:RGB, range:FULL}
Bits per color component	8
Bits per pixel	24
Video Acceleration Information
Log Messages
[2546:2546:0426/092802.155153:ERROR:sandbox_linux.cc(379)] : InitializeSandbox() called with multiple threads in process gpu-process.

There are many ways in which this can be fixed. Some that I though of right now are:

* reduce the value of kMaxTimeupdateEventFrequency and fire it more often (giving some possibility to make it in allowed range when slightly delayed) (ugly and bad, so I wouldn't recommend it)

* refactor it to run these schedules on separate task runner/thread which won't be blocked by heavy tasks to avoid long delays