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Bug 12062 - UTF-8 BOM should not be forbidden in Polyglot Markup
Summary: UTF-8 BOM should not be forbidden in Polyglot Markup
Status: CLOSED FIXED
Alias: None
Product: HTML WG
Classification: Unclassified
Component: LC1 HTML/XHTML Compatibility Authoring Guide (ed: Eliot Graff) (show other bugs)
Version: unspecified
Hardware: PC All
: P2 major
Target Milestone: ---
Assignee: Eliot Graff
QA Contact: HTML WG Bugzilla archive list
URL: http://dev.w3.org/html5/html-xhtml-au...
Whiteboard:
Keywords:
Depends on:
Blocks:
 
Reported: 2011-02-14 13:31 UTC by Leif Halvard Silli
Modified: 2011-08-04 05:07 UTC (History)
7 users (show)

See Also:


Attachments

Description Leif Halvard Silli 2011-02-14 13:31:30 UTC
The spec says:

]]  When polyglot markup uses UTF-8, it does not include a BOM.  [[

I recomment to delete the above statement. Because I can see no basis for it. In my view, the UTF-8 BOM can *help* working with polyglot markup. Justificaiton:

 a) the UTF-8 BOM is understood by both XML and HTML5 parsers;
 b) the UTF-8 BOM allows you to not use <meta> @http-equiv="content-type"
      or @charset (which, nevertheless, only HTML parsers know)

For offline parsing via file:// URLs, the presence of an UTF-8 BOM seems to me as an advantage. For online parsing it also offers the advantage that it provides encoding information even if HTTP fails to provide such information.

The fact that some (very) legacy user agents may act up if they see the UTF-8 BOM has not prevented HTML5 from permitting it. Thus, if the UTF-8 BOM should be declared as something that is not used in Polyglot Markup, then please provide a justification/principle for such a decision. 

Further more, the following statement from the same sections seems to contradict the statement that the UTF-8 BOM should not be used:

]]
Polyglot markup declares character encoding one of two ways:
   By using the BOM.
   In the HTTP header of the response [HTTP11], as in the following:
[[

If you accept my argument that the UTF-8 BOM can be used, then I suggest replacing the above quote with following, more accurate reformulation:

]]
Polyglot markup declares character encoding via the following ways, that might be used separately or in combination, as long as they contains the same encoding information:
   Inside the document: 
     * by the use of a BOM;
     * by relying of the  XML UTF-8 encoding default in combination with <meta charset="UTF-8"/>
   In the HTTP header [ etc - keep the current text ]
[[
Comment 1 Eliot Graff 2011-03-02 23:41:55 UTC
The Editor's Draft of 3 March has the following for section 3, Specifying a Document's Character Encoding:

]]
Polyglot markup uses either UTF-8 or UTF-16. UTF-8 is preferred. When polyglot markup uses UTF-16, it includes the BOM indicating little-endian UTF-16 or big-endian UTF-16. 

Polyglot markup declares character encoding in the following ways, which may be used separately or in combination (If used in combination, each approach contains identical encoding information): 
  Within the document
    &#9702;By using the BOM.
    &#9702;By relying on the default XML UTF-8 encoding in combination with the use of the <meta charset="UTF-8"/> element.
  In the HTTP header of the response [HTTP11], as in the following: 
...(rest unchanged)
[[

I believe these changes satisfy the requests in this bug and have therefore resolved it as fixed. Thanks, again, for your continued help.

Eliot
Comment 2 Leif Halvard Silli 2011-03-03 01:48:42 UTC
(In reply to comment #1) Some nitty gritties. 

FIRSTLY: In the *below* quote, inside the parenthesis, please change the upper-case "I"  (If used ...") to lowercase:

]]
> Polyglot markup declares character encoding in the following ways, which may be
> used separately or in combination (If used in combination, each approach
> contains identical encoding information): 
[[

SECONDLY, in the *below* quote, you changed my  suggested wording from " XML UTF-8 encoding default" to "default XML UTF-8 encoding". I don't know if it is really correct to say that UTF-8 is  _the_ default encoding of XML? My intention was to say that UTF-8 is _an_ encoding default - one of two - the other being UTF-16. (In my proposal, "XML" is an adjective - think "XML-ish", or "of XML" or "XML's".)

>     &#9702;By relying on the default XML UTF-8 encoding in combination with the
> use of the <meta charset="UTF-8"/> element.

It is true that XML says that if a document *DOES NOT*  have an encoding declaration (internal or external) and also does not have an encoding signature (aka BOM), then the document *MUST* be in the UTF-8 encoding - see section '4.3.3 Character Encoding in Entities' of XML 1.0.) From that angle it seems correct that UTF-8 has something to do with "the default"

* But I still think that my wording was better. Feel free to go back and use it.
* Or else, I suggest to use the follwing formulation instead, where I use the word 'autodetection':

]] By relying on XML's autodection of the UTF-8 encoding, in combination with the HTML <meta charset="UTF-8"/> encoding declaration. [[

Justification for the 'autodetection' variant: 

* XML 1.0 has an entire section about 'autodetection of character encodings': http://www.w3.org/TR/REC-xml/#sec-guessing
* in contrast, the word 'default' only occurs once in relation to encoding: http://www.w3.org/TR/REC-xml/#charencoding
* the autodetection section relates to 'default' by the 'other' option in the "Without a Byte Order Mark" table. Quot:
   ]] Other	   UTF-8 without an encoding declaration, or else the data stream is mislabeled
      (lacking a required encoding declaration), corrupt, fragmentary, or enclosed in a wrapper 
     of some kind [[

Sidenote: In Norwegian, 'default' and 'automatic' are often synonyms.
Comment 3 Leif Halvard Silli 2011-03-03 02:18:02 UTC
(In reply to comment #2)
> (In reply to comment #1) Some nitty gritties. 

> SECONDLY, 

> >     &#9702;By relying on the default XML UTF-8 encoding in combination with the
> > use of the <meta charset="UTF-8"/> element.

> * Or else, I suggest to use the follwing formulation instead, where I use the
> word 'autodetection':
> 
> ]] By relying on XML's autodection of the UTF-8 encoding, in combination with
> the HTML <meta charset="UTF-8"/> encoding declaration. [[

Or, replace 'autodetection' with 'default to' or 'fallback to'  - 'fallback to' gives me myself more meaning, but 'default to' might be more computer-ishy:

]] By relying on XML's fallback to the UTF-8 encoding, in combination with
   the HTML <meta charset="UTF-8"/> encoding declaration. [[

Eventually

]] By relying on UTF-8 as the encoding default of XML, in combination with
   the HTML <meta charset="UTF-8"/> encoding declaration. [[

Or something like that. English is funny. :)
Comment 4 Leif Halvard Silli 2011-03-03 11:51:07 UTC
(In reply to comment #1)

> ]]

>   Within the document
>     &#9702;By using the BOM.

> [[

More nitty gritty: you say "the BOM". However there are 3 BOMs. That's the reason why my proposed text said "a BOM".  I suggest that you say 

    ]] ... a BOM[[ 

Or, if you prefer "the BOM", then you must add something - for example 

  ]] By using the BOM _of the specific encoding_ [[

or something. Btw, the BOM is, according to XML, an 'encoding signature'. So that wording is also an option:


  ]] By using the encoding signature (the BOM) of the specific encoding [[
Comment 5 Eliot Graff 2011-03-03 18:29:43 UTC
Thanks, Leif.

The Editor's Draft of 3 March now says:

]]
Polyglot markup uses either UTF-8 or UTF-16. UTF-8 is preferred. When polyglot markup uses UTF-16, it includes the BOM indicating little-endian UTF-16 or big-endian UTF-16.

Polyglot markup declares character encoding in the following ways, which may be used separately or in combination (if used in combination, each approach contains identical encoding information):

    * Within the document
          o By using the BOM of the specific encoding.
          o By relying on UTF-8 as the encoding default of XML, used in combination with the HTML <meta charset="UTF-8"/> element.
    * In the HTTP header of the response [HTTP11], as in the following: 
[[

I believe this takes care of the points you raised.

As they say, the devil is in the details. And if English is funny, it's hysterical when I get to it. :-)

Thanks for your help.

Eliot
Comment 6 Leif Halvard Silli 2011-03-03 22:46:59 UTC
(In reply to comment #5)

I'm very satisfied with your solution. However, I would like to add some more issues relating to the section we are discussing. 

ISSUE 1:

Draft says: ]] polyglot markup does not use <meta content=text/html; charset>[[

Please type a valid http-equiv meta element:   <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />

ISSUE 2 (which evenetually cancels ISSUE 1): You could simply delete this sentence: ]] However, because the mime-type is not necessarily text/html, polyglot markup does not use <meta content=text/html; charset>.]]

Justification: I think I am the source for the above sentence. However, it seems to be to be "over-thought" or "too smart". It only seeks to explain why HTML5 does not permit the http-equiv="content-type" element, and the explanatio sounds logical enough. However, it is, perhaps, not for us to _speculate_ about why it is not permitted? Do as you wish. But I think I would have deleted it.

ISSUE 3 - this is more substantial:

Draft says: ]] when polyglot markup uses UTF-16, it includes the BOM indicating little-endian UTF-16 or big-endian UTF-16 [[

The above quote has two problems:
  PROBLEM I: you say "the BOM". Please say "a BOM".
  PROBLEM II: please say "MUST" rather than "it includes". The MUST is taken from XML 1.0 - which says that a BOM is required when UTF-16 is used.

SOLUTION: This is the reformulation that I suggest:

]] when polyglot markup uses UTF-16, a BOM (indicating little-endian UTF-16 or big-endian UTF-16) MUST be used. [[

(I don't know if you want to point to XML 1.0  with regard to the MUST: http://www.w3.org/TR/REC-xml/#charencoding .)
Comment 7 Leif Halvard Silli 2011-03-03 23:26:07 UTC
(In reply to comment #6)
> (In reply to comment #5)

> ISSUE II

> However, it is, perhaps, not for us to _speculate_ about why it is not permitted? 

The above sentence refers to what HTML5 says. HTML5 says that <meta charset="UTF-8"/> is permitted inside XHTML5. THat is: HTML5 permits that a HTML5 element, is used inside XHTML5 - despite that it is is useless in XHTML5.

Strictly speaking, HTML5 could have permitted other variants of the <meta charset="*"/> element ( as well as variants of the <meta http-equiv="content-type" content="*"/>) - too - it would ease transition between XML and HTML, when non-UTF-8 is used. 

But HTML5 doesn't do that.  And perhaps we don't need to speculate about why. There is a justification inside HTML5: moving documents to and from HTML and XML.

Strictly _strictly_ speaking, I think that when HTML5 _permits_ <meta charset="UTF-8" /> inside XHTML, then it does - in reality - actually _forbid_ other values than "UTF-8" inside the XHTML encoding. (Though - it depens on how one looks at it  - of course.)
Comment 8 Eliot Graff 2011-03-04 19:59:01 UTC
Per comment 6, ISSUE 2, the following sentence is deleted: 
]] 
However, because the mime-type is not necessarily text/html, polyglot markup does not use <meta content=text/html; charset>.
[[

This negates comment 6, ISSUE 1.

For comment 6, ISSUE 3 the draft now says:
]]
When polyglot markup uses UTF-16, it must include a BOM indicating little-endian UTF-16 or big-endian UTF-16, per XML, Character Encoding in Entities. [XML10]
[[

With a link to Character Encoding in Entities from XML 1.0.

Correct me if I am wrong, but I do not see any action required on my part for comment 7.

Thanks so much!

Eliot
Comment 9 Leif Halvard Silli 2011-03-04 23:00:41 UTC
(In reply to comment #8)

> For comment 6, ISSUE 3 the draft now says:
> ]]
> When polyglot markup uses UTF-16, it must include a BOM indicating
> little-endian UTF-16 or big-endian UTF-16, per XML, Character Encoding in
> Entities. [XML10]
> [[
> 
> With a link to Character Encoding in Entities from XML 1.0.

I must admit an error: You were correct in using "the BOM". 

Because, the BOM is not a "mark", it is a character: U+FEFF. And since it is a character, it is of course also a character 'of the specific encoding' used. Therefore it should also not be necessary to add 'of the specific encoding' - in contrast to what I earlier said.

Two consequences/proposals there of:

1) Revert "a BOM" to "the BOM", like you had it.
2) Delete the "of the specific encoding" from 
     ]]By using the BOM of the specific encoding.[[
3) I would also sugget to simply the statement about how to use the UTF-16 encoding- while it is correct that on must use the BOM to indicate little/big endianess, I think you can just state that one must use the BOM - and nothing more. Thus I propose to change the above

]]  When polyglot markup uses UTF-16, it must include a BOM indicating
 little-endian UTF-16 or big-endian UTF-16, per XML, Character Encoding in
 Entities. [XML10]  [[
Comment 10 Eliot Graff 2011-03-04 23:26:14 UTC
I made the changes and published the updated version.

Anything else? ;-)

Thanks, and have a great weekend.

Eliot
Comment 11 Leif Halvard Silli 2011-03-05 00:49:46 UTC
(In reply to comment #10)
> I made the changes and published the updated version.

Thanks.

> Anything else? ;-)

Yes, of course. ;-)

1) I noticed that HTML5 uses the phrase "Byte Order Character character". Also, HTML5 - and all other specs that talk about the BOM - usually introduces it with its un-abbeviated name before it introduces the BOM-abbreviation. Currently, the Polyglot Markup draft does not explain what BOM means. Therefore I suggest to 

a) say  "Byte Order Mark (BOM) character" the first time that the Polyglot spec refers to the BOM:
   ]] When polyglot markup uses UTF-16, it must include the Byte Order Mark (BOM) character[[

b) when possible, try to add the word "character" after each occurrence of 'BOM' or 'Byte Order Mark'

2) After the sentence 'When polyglot markup uses UTF-16 [snip]", then please add this sentence:
    ]] The BOM character can also be used with the UTF-8 encoding. [[

2) Add this comment at the end of/after the paragraph which begins 'Using <meta charset="*"/> has no effect in XML.":

]] The BOM character can be said to be the most polyglot encoding declaration: Unlike the encoding declaration inside the HTTP Content-Type header, it isn't part of a construct that differs for XML and HTML. And unlike the <meta charset="UTF-8"/> declaration of HTML and the UTF-8 encoding default of XML, the BOM character works in both XML and HTML.[[

Somewhere you could also add a link to HTML5's permission to use the BOM character: http://dev.w3.org/html5/spec/syntax.html#writing

Good weekend. ;-)
Comment 12 Eliot Graff 2011-03-11 18:20:16 UTC
Made the edits requested in comment 11 and then further edited to remove UTF-16, per bug 12242. Section 3 now reads:

]]
Polyglot markup declares character encoding in the following ways, which may be used separately or in combination (if used in combination, each approach contains identical encoding information): 
Within the document
&#9702;By using the Byte Order Mark (BOM) character (preferred).
&#9702;By relying on UTF-8 as the encoding default of XML, used in combination with the HTML <meta charset="UTF-8"/> element.
In the HTTP header of the response [HTTP11], as in the following: 
Content-type: text/html; charset=utf-8
 Note that polyglot markup may use either text/html or application/xhtml+xml for the value of the content type. 


Using <meta charset="*"/> has no effect in XML. Therefore, polyglot markup may use <meta charset="*"/> provided the document is encoded as UTF-8 and the value of charset is a case-insensitive match for the string "utf-8". 

Polyglot markup uses UTF-8 encoding. The BOM character may be used with the UTF-8 encoding (see Writing HTML documents in [HTML5]), and using the BOM character is preferred to not using the BOM character. Because the construct of the BOM character is the same for XML and HTML (unlike the encoding declaration inside the HTTP Content-Type header), and because the BOM character works in both XML and HTML (unlike the <meta charset="UTF-8"/> declaration of HTML and the UTF-8 encoding default of XML), the BOM character can be said to be the most polyglot encoding declaration. 

The W3C Internationalization (i18n) Group recommends to always include a visible encoding declaration in a document, because it helps developers, testers, or translation production managers to check the encoding of a document visually. 
[[

I believe this satisfies all the requests in this bug, so, once again, I am resolving it as fixed. I have faith that you will let me know if there are any other issues.
Comment 13 Leif Halvard Silli 2011-03-12 21:26:04 UTC
(In reply to comment #12)

Some things seems like repetition. And the text doesn't fully reflect the fact that UTF-8 now is the only encoding - that changes a few things. And my many suggestions have made it  a bit long. So, once more:

FIRST, I think you should say, as the very first thing, that UTF-8 is the encoding of polyglot markup. So before the very first paragraph ("Polyglot markup declares character encoding in the following ways"), I suggest saying this  (stealing thoughts from bug 12242):

]] 
  Polyglot markup uses the UTF-8 encoding, the only encoding that both HTML and XML parsers are REQUIRED to support. For HTML, the UTF-8 encoding MUST be declared to avoid that user agents defaults to the locale encoding. For XML, then UTF-8 is the encoding default and as such MAY be be left undeclared.
[[

> ]]
> Polyglot markup declares character encoding in the following ways, which may be
> used separately or in combination (if used in combination, each approach
> contains identical encoding information): 
> Within the document
>     &#9702;By using the Byte Order Mark (BOM) character (preferred).
>     &#9702;By relying on UTF-8 as the encoding default of XML, used in combination
>                   with the HTML <meta charset="UTF-8"/> element.

I suggest referring to that element as "the HTML encoding declaration". Hence, reformulation of last sentence:

]]
 &#9702;By relying on UTF-8 as the encoding default of XML, used in 
   combination with the HTML encoding declaration: <meta charset="UTF-8"/>
[[

> Within the document
>     &#9702;By using the Byte Order Mark (BOM) character (preferred).
>     &#9702;By relying on UTF-8 as the encoding default of XML, used in combination
> with the HTML <meta charset="UTF-8"/> element.

<questionmark>
> In the HTTP header of the response [HTTP11], as in the following: 
>     Content-type: text/html; charset=utf-8
>  Note that polyglot markup may use either text/html or application/xhtml+xml
>  for the value of the content type. 
</questionmark>

In the introduction, you say: 
   ]] Other permissible MIME types are text/xml, application/xml, and any MIME type whose subtype ends with the four characters "+xml". [[ 
Thus this note, which limits the mime type to just two, does not reflect the introduction. 

As well: In the name of "Show. Don't tell.", I suggest stating the HTTP section, like so (important to show an example for application/xhtml+xml as well):

]]
In the HTTP header of the response [HTTP11]: 
     &#9702; For HTML: Content-type: text/html; charset=utf-8
     &#9702; For XHTML: Content-type: application/xhtml+xml; charset=utf-8
     (And the same pattern for other pemissable MIME types, see the Introduction.)
[[

> Using <meta charset="*"/> has no effect in XML. Therefore, polyglot markup may
> use <meta charset="*"/> provided the document is encoded as UTF-8 and the value
> of charset is a case-insensitive match for the string "utf-8". 

The phrase "provided the document is encoded as UTF-8 ' does not make sense now that UTF-8 is the only encoding of polyglot markup. How about this remake:

    ]]
 NOTE: Unlike using the BOM character, the HTML encoding declaration (<meta charset="UTF-8"/>) has no effect in XML. But because the UTF-8 encoding is the encoding default of XML, it represents accurate information and can be used. 

   [ And perhaps the note about what the i18n Group's recommendation to always include a visible declaration should be moved up here? ]

    [[

<delete>
> Polyglot markup uses UTF-8 encoding. 
</delete>

This would be a repetition, given that I suggested to say this as the very first thing - see above.

<delete>
> The BOM character may be used with the UTF-8 encoding 
>(see Writing HTML documents in [HTML5]), and using the BOM
> character is preferred to not using the BOM character.
</delete>

As UTF-8 is the only encoding, "may be used with the UTF-8 encoding " is not necessary to say. Did you mean to say "can be used"? May be we can just delete it? In the beginning of this section, BOM chareacter is listed. Hence, it does not feel necessary to repeat here that it can b used.

<delete>
> Because the construct of the BOM character is the same for XML
> and HTML (unlike the encoding declaration inside
> the HTTP Content-Type header) and because the BOM character works in
> both XML and HTML (unlike the <meta charset="UTF-8"/> declaration of 
> HTML and the UTF-8 encoding default of XML), 
> the BOM character can be said to be the
> most polyglot encoding declaration. 
</delete>

By adding "Note: Unlike using the BOM character" above, I think the above lines can be deleted as well. And the point (which I made) that the MIME type differs, can now - if you add application/xhtml+xml as I suggested above, be spotted in the "In the HTTP header of the response [HTTP11]" list.

> The W3C Internationalization (i18n) Group recommends to always include a
> visible encoding declaration in a document, because it helps developers,
> testers, or translation production managers to check the encoding of a document
> visually. 
> [[
Comment 14 Leif Halvard Silli 2011-03-13 02:40:43 UTC
(In reply to comment #13)
As one piece of text - and with some additional changes. Justification etc at the bottom.

]]
3. Specifying a Document's Character Encoding

Polyglot markup uses UTF-8, the only character encoding for which both HTML and XML requires support. For HTML, then UTF-8 has to be explicitly declared, to avoid fallback to a legacy encoding. For XML, then UTF-8 is the encoding default and as such MAY be left undeclared.

The UTF-8 encoding is declared in the following ways, which can be used together or separately:

* Within the document
  o By using the Byte Order Mark (BOM) character (preferred).
  o By using <meta charset="UTF-8"/> (the HTML encoding 
     declaration).
* Outside the document
  o By adding "charset=utf-8" to the MIME/HTTP Content-Type
     header [HTTP11]: 
    HTML Content-Type example: 
        Content-type: text/html; charset=utf-8
    XHTML Content-Type example:
        Content-type: application/xhtml+xml; charset=utf-8

 NOTE: The HTML encoding declaration has no effect in XML. So when this is the only encoding declaration, then it is XML's encoding default that makes XML parsers treat it as UTF-8.

  The W3C Internationalization (i18n) Group recommends to always include a visible encoding declaration in a document, because it helps developers, testers, or translation production managers to check the encoding of a document visually.
[[

JUSTIFICATION for some of the wording choices above:

* 'XML encoding declaration' is a wording used in XML 1.0. 
   'HTML encoding declaration' is made on the same pattern.
* Tried to use '_character_ encoding' at least once.
* 'legacy encoding' = HTML5 uses this wording about 
   non-UTF-8 encodings
* deleted "(if used in combination, each approach contains 
  identical encoding information)" because it is unrelevant when
  the only encoding is UTF-8
* 'Outside the document' - for analogy with your 'Inside the 
  document'
* Important to have both an XHTML exampe and a HTML example 
  with regard to the MIME/HTTP
* Added 'MIME' as, that is what it is.
* Tried to diminish the number of places where the text 
   mentioned the encoding default of XML ...
* 'in combination' does not make good sense as it indicates that
   the methods cooperates. Tried 'together' instead.
* Deleted note about other MIME types because the text says "By adding "charset=utf-8" to the MIME/HTTP Content-Type header", which is valid for any MIME type. The examples are just examples.
* Tried to be short.
Comment 15 Eliot Graff 2011-03-17 20:12:36 UTC
I believe I have everything in the last two comments. The editor's draft of 17 March now has this for section 3:

]]
3. Specifying a Document's Character Encoding

 Polyglot markup uses the UTF-8 character encoding, the only character encoding for which both HTML and XML require support. HTML requires UTF-8 to be explicitly declared to avoid fallback to a legacy encoding [HTML5]. For XML, UTF-8 is an encoding default. As such, character encoding may be left undeclared in XML with the result that UTF-8 is still supported [XML10]. 

Polyglot markup declares the UTF-8 character encoding in the following ways, which may be used separately or in combination: 
Within the document
  &#9702;By using the Byte Order Mark (BOM) character (preferred).
  &#9702;By using <meta charset="UTF-8"/> (the HTML encoding declaration).
Outside the document 
  &#9702;By adding "charset=utf-8" to the MIME/HTTP Content-Type header [HTTP11], as the following examples show in HTML and XML, respectively:
 Example
Content-type: text/html; charset=utf-8
 Example
Content-type: application/xhtml+xml; charset=utf-8

Note
 The HTML encoding declaration has no effect in XML. When the HTML encoding declaration is the only encoding declaration, the encoding default from XML makes XML parsers treat content as UTF-8. 

The W3C Internationalization (i18n) Group recommends to always include a visible encoding declaration in a document, because it helps developers, testers, or translation production managers to check the encoding of a document visually. 
[[

Thanks so much,

Eliot
Comment 16 Leif Halvard Silli 2011-03-17 20:30:50 UTC
Perfect!
Comment 17 Michael[tm] Smith 2011-08-04 05:06:56 UTC
mass-move component to LC1
Comment 18 Michael[tm] Smith 2011-08-04 05:07:20 UTC
mass-move component to LC1