No proper checking with using quotes.

[b01]

I can write invalid CSS like this and LESS will still compile:

.test {
    background: url('/test-image.png); filter(Opacity=50 Aplha');
    /* Notice the missing quote after .png. But there is one after the Alpha, so no error is thrown. */
}

@images: 'test/';
.test2 {
    background: url({@images}/test-image.png);
    /* @{image} will not be replaced because it is not in quotes. However no error will be thrown. */
}

Output:

.test {
  background: url('/test-image.png); filter(Opacity=50 Aplha');
  /* Notice the missing quote after .png. But there is one after the Alpha, so no error is thrown. */
}
.test2 {
  background: url({@images}/test-image.png);
  /* @{image} will not be replaced because it is not in quotes. However no error will be thrown. */
}

Not a big deal, but would help if LESS threw errors on this stuff.

[lukeapage]

That's valid css, its just a bad href. We don't validate urls, that's pretty tricky.

[seven-phases-max]

Yep, I also think the first example is expected to compile fine (the url there is just /test-image.png); filter(Opacity=50 Aplha never validated to not burden the compiler). So the recommendation is just to not put those statements on the same line to not step into such typo error.

The second example is probably not that evident. It is questionable if Less should treat unquoted @var inside urls as a variable, but currently it does not (only quoted "@{vars}" are expanded) so all those { and @ (and whatever else characters) are considered to be a part of the url thus it is again expected that no error thrown (simply because the code has no errors).

[lukeapage]

yes we recommend people always quote URL's so the second point I think is fine.

As for validating URL's that is a good example of something that could be implemented as a plugin.
I raised a feature request on the plugins repo
less/less-plugin-inline-urls#2

[b01]

@lukeapage Thanks for looking at this, you're right.

1. The property has ended after the semi-colon. But it thinks it is part of the string.

background: url('/test-image.png); opacity: 0.5; filter(Opacity=50 Aplha');

1. It thinks the quote is closed, but after it skips some properties.

Now that I think about it, I think that LESS did the right thing here. Because if I forget to put an end quote, I cannot expect that LESS can figure that out. It just reads until it finds the end quote.

Also, while validating URLs may catch this; I don't think LESS should validate URLs. It is the developers fault if they type it wrong. I don't want my compiler wasting time trying to figure that out.