CSS tuple index out of range when parsing flex: ;

[ThiefMaster]

  File "indico/modules/receipts/util.py", line 162, in create_pdf
    css = CSS(string=f'{css}{DEFAULT_CSS}', url_fetcher=css_url_fetcher)
  File "__init__.py", line 296, in __init__
    preprocess_stylesheet(
  File "weasyprint/css/__init__.py", line 887, in preprocess_stylesheet
    declarations = list(preprocess_declarations(
  File "weasyprint/css/validation/__init__.py", line 118, in preprocess_declarations
    result = list(expander_(base_url, name, tokens))
  File "weasyprint/css/validation/expanders.py", line 94, in generic_expander_wrapper
    for new_name, new_token in result:
  File "weasyprint/css/validation/expanders.py", line 603, in expand_flex
    line, column = tokens[0].source_line, tokens[0].source_column

Easy to reproduce with a stylesheet like this:

.foo {
  flex: ;
}

[liZe]

Hi,

Really funny bug, thanks for the report! The problem happens with many shorthand properties.

[liZe]

The problem happens with many shorthand properties.

And some properties too. So strange that nobody ever complained about this!

And even stranger: browsers don’t show the same behaviour. Chrome and Firefox mark the rule as invalid, but Safari seems to accept the empty declaration.

Time to read the specification carefully!

[liZe]

Time to read the specification carefully!

As far as I can tell, empty values are allowed by grammar, but no property or shorthand allows them.