Valid content-type for XML, HTML and XHTML documents

Decoding MIME types

Mime types: The basics

Content-Type is a vital HTTP header defining the type of the data. Three major types exist:

• HTML (HyperText Markup Language) is served with text/html.
• XHTML (eXtensible HyperText Markup Language) can use either application/xhtml+xml for modern applications or text/html for fallback.
• XML (eXtensible Markup Language) usually takes application/xml. Lesser-used subtype is text/xml.

Unraveling MIME types with +xml

Note that MIME types ending in +xml indicate XML-related formats. Whether you come across a customized application/custom+xml or a registered image/svg+xml, understand them as XML subtypes.

Choosing the correct content type

HTML, XHTML, XML: What's in the content-type?

Browsers understand Content-Type, which influences rendering:

• HTML: With text/html, browsers interpret HTML tags and narrate the web story.
• XML: For application/xml, the browser swaps to a parsing model, presenting raw XML if styled else displaying a tree view.
• XHTML: For application/xhtml+xml, non-adherence to markup rules may lead to non-rendering, insisting on enhanced XML rules.

Breaching legacy compatibility

Older systems might not support recent formats:

• Old-school browsing: Deescalate to text/html when serving XHTML for compatibility.
• Content diplomacy: Use HTTP headers to select the best content type hinging on the client-side potential.

Nitty-gritty about edge cases

Deal with specific scenarios:

• Encodings: Always append character encoding, such as UTF-8, to circumvent display issues. For instance, Content-Type: text/html; charset=UTF-8.
• MIME sniffing risks: Explicit content-type safeguard against MIME-Type sniffing vulnerabilities.

Best practices and common pitfalls

The dark side of improper MIME types

Serving inconsistent Content-Type headers triggers:

• Security risks: Browsers may mistakenly interpret resources leading to execution of malicious scripts (XSS).
• Old routine: Incorrect Content-Type fires older HTML rendering modes in browsers that could mar the user experience.

Ensuring smooth content-type delivery

For uninterrupted resource handling:

• Test before delivery: Validate your document and ascertain that the served MIME type aligns with its content.
• Leverage tools: W3C's validator and other tools can test document types saving time.
• HTTP header checks: Developer tools on modern browsers can inspect HTTP headers to confirm if Content-Type is dispatched correctly.