Meta Charset First-1024-Bytes Check

Checks a supplied meta charset declaration and its exclusive ending byte offset against the first 1024 bytes rule. Byte positions must come from the original encoded HTML, not UTF-16 character offsets.

Description

An HTML meta charset declaration must specify UTF-8 and fit entirely within the initial 1024 bytes.

An HTML meta charset declaration must specify UTF-8 and fit entirely within the initial 1024 bytes. Byte position in encoded source differs from character position in a JavaScript string.^[1]

The tool returns utf8, withinFirst1024, passes. The fields describe the supplied local pattern or effective state, or contain the formatted markup shown below.

When to use Meta Charset First-1024-Bytes Check

Use this for the following task: Checks a supplied meta charset declaration and its exclusive ending byte offset against the first 1024 bytes rule. Byte positions must come from the original encoded HTML, not UTF-16 character offsets. Provide the requested local fields extracted from the relevant element or attribute.

How to use Meta Charset First-1024-Bytes Check and understand its results

Prepare the parsed element names, attribute values or direct element-child lists requested below. Keep attribute presence separate from its value.
Enter charset, tagEndByteExclusive according to the field table, then run the tool.
Read utf8, withinFirst1024, passes. Check the effective state, recognized-token flags or generated markup against the rules below.
Correct any reported local mismatch and check the resulting markup in its intended document and browser context.

Input field	Meaning	Value type and bounds	Requirement
charset	Charset	string; maximum text length 100	Required
tagEndByteExclusive	Tag End Byte Exclusive	integer; minimum 0; maximum 9007199254740991	Required

Provide the parsed charset value and the exclusive ending byte offset of the declaration in the original encoded document. Return the two checks and their conjunction.

passes = charset equals utf-8 under ASCII case folding AND exclusive tag end≤1024

The symbols and boundary conditions refer to the supplied fields and the implementation described above.

Output field	Meaning	Value type
utf8	Utf8	boolean
withinFirst1024	Within First1024	boolean
passes	Passes	boolean

json

{
  "input": {
    "charset": "UTF-8",
    "tagEndByteExclusive": 128
  },
  "output": {
    "utf8": true,
    "withinFirst1024": true,
    "passes": true
  }
}

Limitations and assumptions

The tool trusts the supplied offset and does not read or encode a document. It does not trim the charset, assess duplicate declarations or resolve transport-header encoding.

The tool does not fetch a page, run a browser or validate an entire HTML document. Malformed tool requests produce input errors; valid requests can still report unrecognized values or failed local checks.

Alternative or Complementary methods

Choose Void Elements List Checker when you need its operation: Checks parsed HTML element names against the 13 current void elements; names are ASCII case insensitive. This does not parse markup or classify obsolete elements. Choose Meta Refresh seconds;url Format for the complementary operation: Builds an escaped meta refresh element from a nonnegative integer delay in seconds and an absolute HTTP(S) URL. The URL is normalized with the portable WHATWG URL helper; relative URLs and fractional-delay parser behavior are outside this builder profile.

Background reference: HTML Standard: Meta Charset First-1024-Bytes Check.

References

HTML Standard: Meta Charset First-1024-Bytes Check — WHATWG

Similar or alternative tools

Void Elements List Checker
Checks parsed HTML element names against the 13 current void elements; names are ASCII case insensitive. This does not parse markup or classify obsolete elements.
Meta Refresh seconds;url Format
Builds an escaped meta refresh element from a nonnegative integer delay in seconds and an absolute HTTP(S) URL. The URL is normalized with the portable WHATWG URL helper; relative URLs and fractional-delay parser behavior are outside this builder profile.