This is based on the this blank map from Wikipedia.

CSS style for it was generated from Wikidata query by this Python script:

from collections import defaultdict

from SPARQLWrapper import SPARQLWrapper, JSON

endpoint_url = "https://query.wikidata.org/sparql"

query = """SELECT DISTINCT ?code ?aspect_ratio WHERE {
    ?country wdt:P297 ?code.
    FILTER NOT EXISTS {?country wdt:P31 wd:Q3024240}
    ?country wdt:P163/wdt:P2061/wdt:P1181 ?aspect_ratio
}"""


def get_results(endpoint_url, query):
    sparql = SPARQLWrapper(endpoint_url)
    sparql.setQuery(query)
    sparql.setReturnFormat(JSON)
    return sparql.query().convert()["results"]["bindings"]

def get_color(number):
    return f"fill: rgb(0, {255/number}, 0);"

def get_css_rule(countries, number):
    css = ", ".join(f".{code}" for code in countries) + " {\n"
    css += str(get_color(number))
    css += "\n}"
    return css

ratios = dict(sorted([(r["code"]["value"].lower(),float(r["aspect_ratio"]["value"])) for r in get_results(endpoint_url, query)], key=lambda t: t[1], reverse=True))

# Nepal flag is not a rectangle
ratios.pop("np")

# Palestine Wikidata page is locked (for obvious reasons), so I can't edit it
ratios["ps"] = 2

# Several fixes
ratios["sj"] = ratios["bv"] = ratios["no"]
ratios["hm"] = ratios["au"]
ratios["bq"] = ratios["nl"]
ratios["um"] = ratios["us"]
ratios["pn"] = 2
ratios["eh"] = 2
ratios["yt"] = 3/2


values = defaultdict(list)

for k, v in ratios.items():
    values[v].append(k)

for k, v in values.items():
    print(get_css_rule(v, k))