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HTML Parser: confusion about adoption agency algorithm #9559

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stasoid opened this issue Jul 25, 2023 · 9 comments
Open

HTML Parser: confusion about adoption agency algorithm #9559

stasoid opened this issue Jul 25, 2023 · 9 comments
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clarification Standard could be clearer topic: parser

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@stasoid
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stasoid commented Jul 25, 2023 •
edited

https://html.spec.whatwg.org/multipage/parsing.html#adoption-agency-algorithm

What exactly does this text mean:

If there is no such element, then return and instead act as described in the "any other end tag" entry above.

What UA should do after executing steps in "any other end tag"?

  1. Should a call site look like this:
if (adoption_agency(token) == run_any_other_end_tag)
    goto any_other_end_tag;
// further steps will NOT be executed
  1. Or like this:
if (adoption_agency(token) == run_any_other_end_tag)
    any_other_end_tag(token);
// further steps will be executed

Ladybird behaves like 1, Chrome and html5lib - like 2 (they call any_other_end_tag directly from adoption_agency). Firefox, gumbo, and lexbor don't call any_other_end_tag at all (they ignore the return value of adoption_agency).
@annevk annevk added clarification Standard could be clearer topic: parser labels Jul 26, 2023
@annevk
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annevk commented Jul 26, 2023

cc @whatwg/html-parser

@stasoid stasoid mentioned this issue Jul 26, 2023
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@zcorpan
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zcorpan commented Aug 14, 2023

Chromium and html5lib both have a return, so further steps in AAA will not be executed. Assuming a return, is there a difference between 1 and 2?

Do you have a test that shows different output between implementations here?

@stasoid
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stasoid commented Aug 14, 2023

Further steps at AAA call site, not in the AAA algorithm itself. See how it is implemented in Ladybird.

No, I don't have a test.

@zcorpan
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zcorpan commented Aug 14, 2023

The relevant line for Firefox I think is
https://github.com/mozilla/gecko-dev/blob/00e6644d0db8acf9372702324151b8077a3d2bb7/parser/html/nsHtml5TreeBuilder.cpp#L3714

@stasoid
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stasoid commented Aug 14, 2023

If Chrome/html5lib behavior is the correct one, I propose to change the wording to:

If there is no such element, then act as described in the "any other end tag" entry above and return.

I.e. swap "return" and "act as described..." This makes it much clearer what implementation is supposed to do.

@zcorpan
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zcorpan commented Aug 31, 2023

This might be a test for this:

<nobr><nobr>

If the remaining steps at the call site at https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inbody:adoption-agency-algorithm-3 are not run, then the second nobr element would not be inserted. But Chrome/Safari/Firefox do insert it.

@stasoid
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stasoid commented Aug 31, 2023

No, we need AAA step 4.3 to not find formatting element with the tag name subject to trigger the desired behavior. In this case it finds it. (AAA is called on the second <nobr>, so it finds the first <nobr> in the list of active formatting elements.)

@stasoid
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stasoid commented Sep 1, 2023 •
edited

I believe this is the test (found by brute force):

<object><a><p><a>

Update: No, Ladybird tree is the same as Chrome:

<html>
    <head>
    <body>
        <object>
            <a>
            <p>
                <a>
                <a>

@stasoid
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stasoid commented Sep 3, 2023 •
edited

If lexbor behavior is changed to match that of Ladybird, it still passes all the html5lib tests.

If lexbor behavior is changed to match that of Chrome, it still passes all the html5lib tests.

Lexbor author says that "for open tags (A and NOBR) there can be no situation when lxb_html_tree_adoption_agency_algorithm() function returns true" (true == run_any_other_end_tag here).

So there might indeed be no difference between Ladybird and Chrome behavior, but this is not obvious at all.

@stasoid stasoid mentioned this issue Nov 7, 2023
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