diff --git a/ports/webassembly/Makefile b/ports/webassembly/Makefile
index 4e1d53b0cc..e8f27d862b 100644
--- a/ports/webassembly/Makefile
+++ b/ports/webassembly/Makefile
@@ -1,23 +1,67 @@
+################################################################################
+# Initial setup of Makefile environment.
+
+# Select the variant to build for:
+ifdef VARIANT_DIR
+# Custom variant path - remove trailing slash and get the final component of
+# the path as the variant name.
+VARIANT ?= $(notdir $(VARIANT_DIR:/=))
+else
+# If not given on the command line, then default to standard.
+VARIANT ?= standard
+VARIANT_DIR ?= variants/$(VARIANT)
+endif
+
+ifeq ($(wildcard $(VARIANT_DIR)/.),)
+$(error Invalid VARIANT specified: $(VARIANT_DIR))
+endif
+
+# If the build directory is not given, make it reflect the variant name.
+BUILD ?= build-$(VARIANT)
+
 include ../../py/mkenv.mk
+include $(VARIANT_DIR)/mpconfigvariant.mk
 
-CROSS = 0
-
+# Qstr definitions (must come before including py.mk).
 QSTR_DEFS = qstrdefsport.h
 
+# Include py core make definitions.
 include $(TOP)/py/py.mk
 include $(TOP)/extmod/extmod.mk
 
+################################################################################
+# Project specific settings and compiler/linker flags.
+
 CC = emcc
 LD = emcc
+TERSER ?= npx terser
 
 INC += -I.
 INC += -I$(TOP)
 INC += -I$(BUILD)
+INC += -I$(VARIANT_DIR)
 
 CFLAGS += -std=c99 -Wall -Werror -Wdouble-promotion -Wfloat-conversion
 CFLAGS += -Os -DNDEBUG
 CFLAGS += $(INC)
 
+JSFLAGS += -s EXPORTED_FUNCTIONS="\
+	_mp_js_init,\
+	_mp_js_init_repl,\
+	_mp_js_do_str,\
+	_mp_js_process_char,\
+	_mp_hal_get_interrupt_char,\
+	_mp_sched_keyboard_interrupt$(EXPORTED_FUNCTIONS_EXTRA)"
+JSFLAGS += -s EXPORTED_RUNTIME_METHODS="\
+	ccall,\
+	cwrap,\
+	FS$(EXPORTED_RUNTIME_METHODS_EXTRA)"
+JSFLAGS += --js-library library.js
+JSFLAGS += -s SUPPORT_LONGJMP=emscripten
+
+################################################################################
+# Source files and libraries.
+
 SRC_SHARED = $(addprefix shared/,\
 	runtime/interrupt_char.c \
 	runtime/stdout_helpers.c \
@@ -26,33 +70,41 @@ SRC_SHARED = $(addprefix shared/,\
 	timeutils/timeutils.c \
 	)
 
-SRC_C = \
+SRC_C += \
 	main.c \
 	mphalport.c \
 
+# List of sources for qstr extraction.
 SRC_QSTR += $(SRC_C) $(SRC_SHARED)
 
+SRC_JS ?= wrapper.js
+
 OBJ += $(PY_O)
 OBJ += $(addprefix $(BUILD)/, $(SRC_SHARED:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(SRC_C:.c=.o))
 
-JSFLAGS += -s ASYNCIFY
-JSFLAGS += -s EXPORTED_FUNCTIONS="['_mp_js_init', '_mp_js_init_repl', '_mp_js_do_str', '_mp_js_process_char', '_mp_hal_get_interrupt_char', '_mp_sched_keyboard_interrupt']"
-JSFLAGS += -s EXPORTED_RUNTIME_METHODS="['ccall', 'cwrap', 'FS']"
-JSFLAGS += --js-library library.js
+################################################################################
+# Main targets.
+
+.PHONY: all min test
 
 all: $(BUILD)/micropython.js
 
-$(BUILD)/micropython.js: $(OBJ) library.js wrapper.js
-	$(ECHO) "LINK $(BUILD)/firmware.js"
-	$(Q)emcc $(LDFLAGS) -o $(BUILD)/firmware.js $(OBJ) $(JSFLAGS)
-	cat wrapper.js $(BUILD)/firmware.js > $@
+$(BUILD)/micropython.js: $(OBJ) library.js $(SRC_JS)
+	$(ECHO) "LINK $@"
+	$(Q)emcc $(LDFLAGS) -o $@ $(OBJ) $(JSFLAGS)
+	$(Q)cat $(SRC_JS) >> $@
 
-min: $(BUILD)/micropython.js
-	uglifyjs $< -c -o $(BUILD)/micropython.min.js
+$(BUILD)/micropython.min.js: $(BUILD)/micropython.js
+	$(TERSER) $< --compress --module -o $@
+
+min: $(BUILD)/micropython.min.js
 
 test: $(BUILD)/micropython.js $(TOP)/tests/run-tests.py
 	$(eval DIRNAME=ports/$(notdir $(CURDIR)))
 	cd $(TOP)/tests && MICROPY_MICROPYTHON=../ports/webassembly/node_run.sh ./run-tests.py -j1
 
+################################################################################
+# Remaining make rules.
+
 include $(TOP)/py/mkrules.mk
diff --git a/ports/webassembly/mpconfigport.h b/ports/webassembly/mpconfigport.h
index d026136239..abfbbca794 100644
--- a/ports/webassembly/mpconfigport.h
+++ b/ports/webassembly/mpconfigport.h
@@ -25,17 +25,18 @@
  * THE SOFTWARE.
  */
 
+// Options to control how MicroPython is built for this port, overriding
+// defaults in py/mpconfig.h.
+
 #include <stdint.h>
 #include <stdlib.h> // for malloc, for MICROPY_GC_SPLIT_HEAP_AUTO
 
-// options to control how MicroPython is built
+// Variant-specific definitions.
+#include "mpconfigvariant.h"
 
+#ifndef MICROPY_CONFIG_ROM_LEVEL
 #define MICROPY_CONFIG_ROM_LEVEL (MICROPY_CONFIG_ROM_LEVEL_EXTRA_FEATURES)
-
-// You can disable the built-in MicroPython compiler by setting the following
-// config option to 0.  If you do this then you won't get a REPL prompt, but you
-// will still be able to execute pre-compiled scripts, compiled with mpy-cross.
-#define MICROPY_ENABLE_COMPILER     (1)
+#endif
 
 #define MICROPY_ALLOC_PATH_MAX      (256)
 #define MICROPY_READER_VFS          (MICROPY_VFS)
@@ -69,6 +70,13 @@
         mp_handle_pending(true); \
     } while (0);
 
+// Whether the VM will periodically call mp_js_hook(), which checks for
+// interrupt characters on stdin (or equivalent input).
+#ifndef MICROPY_VARIANT_ENABLE_JS_HOOK
+#define MICROPY_VARIANT_ENABLE_JS_HOOK (0)
+#endif
+
+#if MICROPY_VARIANT_ENABLE_JS_HOOK
 #define MICROPY_VM_HOOK_COUNT (10)
 #define MICROPY_VM_HOOK_INIT static uint vm_hook_divisor = MICROPY_VM_HOOK_COUNT;
 #define MICROPY_VM_HOOK_POLL if (--vm_hook_divisor == 0) { \
@@ -78,6 +86,7 @@
 }
 #define MICROPY_VM_HOOK_LOOP MICROPY_VM_HOOK_POLL
 #define MICROPY_VM_HOOK_RETURN MICROPY_VM_HOOK_POLL
+#endif
 
 // type definitions for the specific machine
 
diff --git a/ports/webassembly/variants/standard/mpconfigvariant.h b/ports/webassembly/variants/standard/mpconfigvariant.h
new file mode 100644
index 0000000000..7be62ea7f4
--- /dev/null
+++ b/ports/webassembly/variants/standard/mpconfigvariant.h
@@ -0,0 +1 @@
+#define MICROPY_VARIANT_ENABLE_JS_HOOK (1)
diff --git a/ports/webassembly/variants/standard/mpconfigvariant.mk b/ports/webassembly/variants/standard/mpconfigvariant.mk
new file mode 100644
index 0000000000..62ee161907
--- /dev/null
+++ b/ports/webassembly/variants/standard/mpconfigvariant.mk
@@ -0,0 +1 @@
+JSFLAGS += -s ASYNCIFY