Minimalist makefile using PostCSS

makefile

# some variables
POSTCSS = ./node_modules/.bin/postcss
POSTCSS_FLAGS = --use autoprefixer autoprefixer.browsers "> 2%"
# wildcard expansion is performed only in targets and in prerequisites so here we need $(wildcard)
SOURCES = $(wildcard src/css/*.css)
# use $(SOURCES) to determine what we actually need using a bit of pattern substitution
TARGETS = $(patsubst src%, build%, $(SOURCES))

# our default target (see below)
all: $(TARGETS)

# as described halfway through https://www.thumbtack.com/engineering/makefiles-for-less-and-css/
# current make needs an explicit target such as "all" when there is only one build target
build/css/%.css: src/css/%.css
	@echo "$< => $@"
	@$(POSTCSS) $(POSTCSS_FLAGS) --output $@ $<

# simply removes $(TARGETS)
clean:
	-@rm -f $(TARGETS)

# both all and clean are not actual filenames
.PHONY: all clean

package.json

{
  "devDependencies": {
    "autoprefixer": "^6.3.1",
    "postcss-cli": "^2.5.0",
  }
}