A script element has several associated pieces of state.

The first is a flag indicating whether or not the script block has been "already started". Initially, script elements must have this flag unset (script blocks, when created, are not "already started"). When a script element is cloned, the "already started" flag, if set, must be propagated to the clone when it is created.

The second is a flag indicating whether the element was "parser-inserted". Initially, script elements must have this flag unset. It is set by the HTML parser and the XML parser on script elements they insert and affects the processing of those elements.

The third is a flag indicating whether the element will "force-async". Initially, script elements must have this flag set. It is unset by the HTML parser and the XML parser on script elements they insert. In addition, whenever a script element whose "force-async" flag is set has a async content attribute added, the element's "force-async" flag must be unset.

The fourth is a flag indicating whether or not the script block is "ready to be parser-executed". Initially, script elements must have this flag unset (script blocks, when created, are not "ready to be parser-executed"). This flag is used only for elements that are also "parser-inserted", to let the parser know when to execute the script.

The last few pieces of state are the script block's type, the script block's character encoding, and the script block's fallback character encoding. They are determined when the script is prepared, based on the attributes on the element at that time, and the Document of the script element.

When a script element that is not marked as being "parser-inserted" experiences one of the events listed in the following list, the user agent must synchronously prepare the script element:

• The script element gets inserted into a document.
• The script element is in a Document and its child nodes are changed.
• The script element is in a Document and has a src attribute set where previously the element had no such attribute.

To prepare a script, the user agent must act as follows:

1. If the script element is marked as having "already started", then the user agent must abort these steps at this point. The script is not executed.

2. If the element has its "parser-inserted" flag set, then set was-parser-inserted to true and unset the element's "parser-inserted" flag. Otherwise, set was-parser-inserted to false.

   This is done so that if parser-inserted script elements fail to run when the parser tries to run them, e.g. because they are empty or specify an unsupported scripting language, another script can later mutate them and cause them to run again.

3. If was-parser-inserted is true and the element does not have an async attribute, then set the element's "force-async" flag to true.

   This is done so that if a parser-inserted script element fails to run when the parser tries to run it, but it is later executed after a script dynamically updates it, it will execute asynchronously even if the async attribute isn't set.

4. If the element has no src attribute, and its child nodes, if any, consist only of comment nodes and empty text nodes, then the user agent must abort these steps at this point. The script is not executed.

5. If the element is not in a Document, then the user agent must abort these steps at this point. The script is not executed.

6. If either:

   • the script element has a type attribute and its value is the empty string, or
   • the script element has no type attribute but it has a language attribute and that attribute's value is the empty string, or
   • the script element has neither a type attribute nor a language attribute, then

   ...let the script block's type for this script element be "text/javascript".

   Otherwise, if the script element has a type attribute, let the script block's type for this script element be the value of that attribute with any leading or trailing sequences of space characters removed.

   Otherwise, the element has a non-empty language attribute; let the script block's type for this script element be the concatenation of the string "text/" followed by the value of the language attribute.

   The language attribute is never conforming, and is always ignored if there is a type attribute present.

7. If the user agent does not support the scripting language given by the script block's type for this script element, then the user agent must abort these steps at this point. The script is not executed.

8. If was-parser-inserted is true, then flag the element as "parser-inserted" again, and set the element's "force-async" flag to false.

9. The user agent must set the element's "already started" flag.

   The state of the element at this moment is later used to determine the script source.

10. If the element is flagged as "parser-inserted", but the element's Document is not the Document of the parser that created the element, then abort these steps.

11. If scripting is disabled for the script element, then the user agent must abort these steps at this point. The script is not executed.

    The definition of scripting is disabled means that, amongst others, the following scripts will not execute: scripts in XMLHttpRequest's responseXML documents, scripts in DOMParser-created documents, scripts in documents created by XSLTProcessor's transformToDocument feature, and scripts that are first inserted by a script into a Document that was created using the createDocument() API. [XHR] [DOMPARSER] [DOMCORE]

12. If the script element has an event attribute and a for attribute, then run these substeps:

    1. Let for be the value of the for attribute.

    2. Let event be the value of the event attribute.

    3. Strip leading and trailing whitespace from event and for.

    4. If for is not an ASCII case-insensitive match for the string "window", then the user agent must abort these steps at this point. The script is not executed.

    5. If event is not an ASCII case-insensitive match for either the string "onload" or the string "onload()", then the user agent must abort these steps at this point. The script is not executed.

13. If the script element has a charset attribute, then let the script block's character encoding for this script element be the encoding given by the charset attribute.

    Otherwise, let the script block's fallback character encoding for this script element be the same as the encoding of the document itself.

    Only one of these two pieces of state is set.

14. If the element has a src attribute whose value is not the empty string, then the value of that attribute must be resolved relative to the element, and if that is successful, the specified resource must then be fetched, from the origin of the element's Document.

    If the src attribute's value is the empty string or if it could not be resolved, then the user agent must queue a task to fire a simple event named error at the element, and abort these steps.

    For historical reasons, if the URL is a javascript: URL, then the user agent must not, despite the requirements in the definition of the fetching algorithm, actually execute the script in the URL; instead the user agent must act as if it had received an empty HTTP 400 response.

    For performance reasons, user agents may start fetching the script as soon as the attribute is set, instead, in the hope that the element will be inserted into the document. Either way, once the element is inserted into the document, the load must have started. If the UA performs such prefetching, but the element is never inserted in the document, or the src attribute is dynamically changed, then the user agent will not execute the script, and the fetching process will have been effectively wasted.

15. Then, the first of the following options that describes the situation must be followed:

    If the element has a src attribute, and the element has a defer attribute, and the element has been flagged as "parser-inserted", and the element does not have an async attribute

    The element must be added to the end of the list of scripts that will execute when the document has finished parsing associated with the Document of the parser that created the element.

    The task that the networking task source places on the task queue once the fetching algorithm has completed must set the element's "ready to be parser-executed" flag. The parser will handle executing the script.

    If the element has a src attribute, and the element has been flagged as "parser-inserted", and the element does not have an async attribute

    The element is the pending parsing-blocking script of the Document of the parser that created the element. (There can only be one such script per Document at a time.)

    The task that the networking task source places on the task queue once the fetching algorithm has completed must set the element's "ready to be parser-executed" flag. The parser will handle executing the script.

    If the element does not have a src attribute, and the element has been flagged as "parser-inserted", and the Document of the HTML parser or XML parser that created the script element has a style sheet that is blocking scripts

    The element is the pending parsing-blocking script of the Document of the parser that created the element. (There can only be one such script per Document at a time.)

    Set the element's "ready to be parser-executed" flag. The parser will handle executing the script.

    If the element has a src attribute, does not have an async attribute, and does not have the "force-async" flag set

    The element must be added to the end of the list of scripts that will execute in order as soon as possible associated with the Document of the script element at the time the prepare a script algorithm started.

    The task that the networking task source places on the task queue once the fetching algorithm has completed must run the following steps:

    1. If the element is not now the first element in the list of scripts that will execute in order as soon as possible to which it was added above, then mark the element as ready but abort these steps without executing the script yet.

    2. Execution: Execute the script block corresponding to the first script element in this list of scripts that will execute in order as soon as possible.

    3. Remove the first element from this list of scripts that will execute in order as soon as possible.

    4. If this list of scripts that will execute in order as soon as possible is still not empty and the first entry has already been marked as ready, then jump back to the step labeled execution.

    If the element has a src attribute

    The element must be added to the set of scripts that will execute as soon as possible of the Document of the script element at the time the prepare a script algorithm started.

    The task that the networking task source places on the task queue once the fetching algorithm has completed must execute the script block and then remove the element from the set of scripts that will execute as soon as possible.

    Otherwise

    The user agent must immediately execute the script block, even if other scripts are already executing.

Fetching an external script must delay the load event of the element's document until the task that is queued by the networking task source once the resource has been fetched (defined above) has been run.

The pending parsing-blocking script of a Document is used by the Document's parser(s).

If a script element that blocks a parser gets moved to another Document before it would normally have stopped blocking that parser, it nonetheless continues blocking that parser until the condition that causes it to be blocking the parser no longer applies (e.g. if the script is a pending parsing-blocking script because there was a style sheet that is blocking scripts when it was parsed, but then the script is moved to another Document before the style sheet loads, the script still blocks the parser until the style sheets are all loaded, at which time the script executes and the parser is unblocked).

When the user agent is required to execute a script block, it must run the following steps:

1. If the element is flagged as "parser-inserted", but the element's Document is not the Document of the parser that created the element, then abort these steps.

2. Jump to the appropriate set of steps from the list below:

   If the load resulted in an error (for example a DNS error, or an HTTP 404 error)

   Executing the script block must just consist of firing a simple event named error at the element.

   If the load was successful

   Executing the script block must consist of running the following steps. For the purposes of these steps, the script is considered to be from an external file if, while the prepare a script algorithm above was running for this script, the script element had a src attribute specified.

   1. Initialize the script block's source as follows:

      If the script is from an external file and the script block's type is a text-based language

      The contents of that file, interpreted as string of Unicode characters, are the script source.

      To obtain the string of Unicode characters, the user agent run the following steps:

      1. If the resource's Content Type metadata, if any, specifies a character encoding, and the user agent supports that encoding, then let character encoding be that encoding, and jump to the bottom step in this series of steps.

      2. If the algorithm above set the script block's character encoding, then let character encoding be that encoding, and jump to the bottom step in this series of steps.

      3. For each of the rows in the following table, starting with the first one and going down, if the file has as many or more bytes available than the number of bytes in the first column, and the first bytes of the file match the bytes given in the first column, then set character encoding to the encoding given in the cell in the second column of that row, and jump to the bottom step in this series of steps:

         Bytes in Hexadecimal	Encoding
         FE FF	Big-endian UTF-16
         FF FE	Little-endian UTF-16
         EF BB BF	UTF-8

         This step looks for Unicode Byte Order Marks (BOMs).

      4. Let character encoding be the script block's fallback character encoding.

      5. Convert the file to Unicode using character encoding, following the rules for doing so given by the specification for the script block's type.

      If the script is from an external file and the script block's type is an XML-based language

      The external file is the script source. When it is later executed, it must be interpreted in a manner consistent with the specification defining the language given by the script block's type.

      If the script is inline and the script block's type is a text-based language

      The value of the text IDL attribute at the time the element's "already started" flag was last set is the script source.

      If the script is inline and the script block's type is an XML-based language

      The child nodes of the script element at the time the element's "already started" flag was last set are the script source.

   2. If the script is from an external file, then increment the ignore-destructive-writes counter of the script element's Document. Let neutralized doc be that Document.

   3. Create a script from the script element node, using the script block's source and the script block's type.

      This is where the script is compiled and actually executed.

   4. Decrement the ignore-destructive-writes counter of neutralized doc, if it was incremented in the earlier step.

   5. If the script is from an external file, fire a simple event named load at the script element.

      Otherwise, the script is internal; queue a task to fire a simple event named load at the script element.

The IDL attributes src, type, charset, and defer, each must reflect the respective content attributes of the same name.

The async IDL attribute controls whether the element will execute asynchronously or not. If the element's "force-async" flag is set, then, on getting, the async IDL attribute must return true, and on setting, the "force-async" flag must first be unset, and then the content attribute must be removed if the IDL attribute's new value is false, and must be set to the empty string if the IDL attribute's new value is true. If the element's "force-async" flag is not set, the IDL attribute must reflect the async content attribute.

IDL 属性 text は、script 要素の直下の子要素となるすべてのテキストノードのコンテンツをツリー順に連結したものを返さなければいけません（コメントや要素は無視されます）。値が指定されたら、textContent IDL 属性と同じように動作しなければいけません。

スクリプト・ブロックのタイプが、ユーザーエージェントが実装しているスクリプト言語の MIME タイプ文字列に一致するなら、ユーザーエージェントはスクリプト言語をサポートしているといいます。大文字・小文字は区別しません。

ユーザーエージェントは、他の MIME タイプや言語をサポートすることができます。

サポートする言語かどうかをタイプから決定するとき、ユーザーエージェントは、未知の MIME パラメータを無視してはいけません。未知のパラメータ付きのタイプは、未対応であると見なさなければいけません。charset パラメータは、ここでの MIME タイプ の比較という意味においては、未知のパラメータとして扱われなければいけません。

4.3.1.4 script 要素と XSLT との相互作用

このセクションは非規定です。

本仕様は、XSLT が script 要素とどう作用し合うのか（実際には、XSLT 処理が stop parsing 手順をどのように呼び出すのか、ナビゲーション・アルゴリズムとどう作用し合うのか、イベント・ループに如何に合わせるのか）については定義しません。しかし、実際にはこれを定義している仕様がないため、ここでは、実装者向けに、既存の実装に基づいて、いくつかのガイドラインを示します：

• XSLT 変形プログラムが <?xml-stylesheet?> 処理命令によって呼び出され、ブラウザーが直接的な DOM 変形を実装しているなら、XSLT プロセッサーによって生成された script 要素は、"parser-inserted" とマークされる必要があります。そして、ドキュメント順に実行する必要があります(modulo scripts marked defer or async)。一方、変形は非同期に行われます。

• XSLTProcessor.transformToDocument() メソッドは、ブラウジング・コンテキストにない Document に要素を追加します。そして、それに応じて、それらが生成するあらゆる script 要素は、スクリプト準備アルゴリズムにおいて、"already started" フラグ がセットされる必要があります。決して実行されることはありません（スクリプティングは無効です。）。そのような script 要素は、なお、"parser-inserted" とマークされている必要があります。しかし、async IDL 属性は async コンテンツ属性が無いため false を返します。

• XSLTProcessor.transformToFragment() メソッドは、document.createElementNS() を使って要素を生成することによって、手動で生成されたものと同等であるフラグメントを生成する必要があります。例えば、それは、"parser-inserted" ではなく、"already started" フラグを持たない script 要素を生成する必要があります。そうすることで、それらは、そのフラグメントがドキュメントに挿入されるときに実行することができます。

最初の 2 つの場合と最後の場合の間の大きな違いは、最初の 2 つは Document で動作するのに対し、最後のはフラグメントで動作するという点です。