Hello, I see that cmake/OpenCVCompilerOptions.cmake has section (line 181 for the current version) :
# We need pthread's
if(UNIX AND NOT ANDROID AND NOT (APPLE AND CV_CLANG)) # TODO
add_extra_compiler_option(-pthread)
endif()

This is what I see during the build:

C/C++: -- Built as dynamic libs?: NO -- C++ standard: 11 -- C++ Compiler: /home/alex/emsdk/upstream/emscripten/em++ (ver 13.0.0) -- C++ flags (Release): -s WASM=1 -s USE_PTHREADS=0 -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winconsistent-missing-override -Wno-delete-non-virtual-dtor -Wno-unnamed-type-template-args -Wno-comment -Wno-deprecated-enum-enum-conversion -Wno-deprecated-anon-enum-enum-conversion -fdiagnostics-show-option **-pthread** -Qunused-arguments -ffunction-sections -fdata-sections -fvisibility=hidden -fvisibility-inlines-hidden -O3 -DNDEBUG -DNDEBUG -- C++ flags (Debug): -s WASM=1 -s USE_PTHREADS=0 -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winconsistent-missing-override -Wno-delete-non-virtual-dtor -Wno-unnamed-type-template-args -Wno-comment -Wno-deprecated-enum-enum-conversion -Wno-deprecated-anon-enum-enum-conversion -fdiagnostics-show-option **-pthread** -Qunused-arguments -ffunction-sections -fdata-sections -fvisibility=hidden -fvisibility-inlines-hidden -g -O0 -DDEBUG -D_DEBUG -- C Compiler: /home/alex/emsdk/upstream/emscripten/emcc -- C flags (Release): -s WASM=1 -s USE_PTHREADS=0 -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winconsistent-missing-override -Wno-delete-non-virtual-dtor -Wno-unnamed-type-template-args -Wno-comment -Wno-deprecated-enum-enum-conversion -Wno-deprecated-anon-enum-enum-conversion -fdiagnostics-show-option **-pthread** -Qunused-arguments -ffunction-sections -fdata-sections -fvisibility=hidden -fvisibility-inlines-hidden -O3 -DNDEBUG -DNDEBUG -- C flags (Debug): -s WASM=1 -s USE_PTHREADS=0 -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winconsistent-missing-override -Wno-delete-non-virtual-dtor -Wno-unnamed-type-template-args -Wno-comment -Wno-deprecated-enum-enum-conversion -Wno-deprecated-anon-enum-enum-conversion -fdiagnostics-show-option **-pthread** -Qunused-arguments -ffunction-sections -fdata-sections -fvisibility=hidden -fvisibility-inlines-hidden -g -O0 -DDEBUG -D_DEBUG -- Linker flags (Release): -Wl,--gc-sections -- Linker flags (Debug): -Wl,--gc-sections -- ccache: NO -- Precompiled headers: NO -- Extra dependencies: -- 3rdparty dependencies: libprotobuf libjpeg-turbo libpng libtiff libopenjp2 zlib quirc

So for the latest emscripten - this is true and it automatically enables pthread and makes 12 section "DataCount" for binary which is not supported by Safari at least.
It should be disabled by default for emscripten.

One more issue inside of build_js.py. It gives error until you replace line:
args = parser.parse_args()
by line:
args, _ = parser.parse_known_args()