In web standards, what is the main difference between HTML and XHTML when designing and validating web pages?

Introduction / Context:
HTML and XHTML are closely related markup languages used to define the structure of web pages. Although they look similar, they follow different rules. This question checks whether you know the key conceptual difference between traditional HTML and XHTML, which is important for writing valid, standards compliant code and for understanding older specifications that many legacy sites still follow.

Given Data / Assumptions:

• Both HTML and XHTML are used to mark up content for web browsers.
• HTML follows its own SGML or HTML specifications depending on version.
• XHTML is defined as a reformulation of HTML using XML rules.
• The question focuses on syntax requirements and validation.

Concept / Approach:
The key difference is that XHTML is designed as an XML application. This means that XHTML documents must be well formed according to XML rules: all tags must be properly nested, attributes must be quoted, and empty elements must be closed explicitly. Traditional HTML is more forgiving and allows certain tags to be omitted or not closed. Therefore, the correct option must mention XML, tighter rules, and well formed syntax.

Step-by-Step Solution:
1. Recall that HTML was originally based on SGML specifications and evolved over time, often allowing loose syntax.2. XHTML emerged as HTML expressed in XML, adding strict requirements such as case sensitivity and proper closing of elements.3. Option A says that XHTML is HTML rewritten as a stricter XML based markup language that requires well formed syntax and proper nesting. This matches the formal definition.4. Option B claims that XHTML is a binary format for images, which is incorrect because both HTML and XHTML are text based.5. Option C incorrectly assigns XHTML to email only and HTML to desktop applications, which has no basis in web standards.6. Option D says there is no difference at all, which ignores the strict XML requirements of XHTML.7. Therefore, Option A is the correct answer.

Verification / Alternative check:
If you examine the official specifications, you will see that XHTML is defined as an XML application. Tools that parse XHTML use XML parsers, which require documents to be well formed. For example, the tag br must be written as br / in XHTML, and attribute values must always be quoted. HTML parsers, in contrast, can accept missing end tags or unquoted attributes and still render the page. This supports the distinction described in Option A.

Why Other Options Are Wrong:
Option B is wrong because neither HTML nor XHTML is a binary image format.Option C is wrong because both HTML and XHTML are used on the web and not restricted to email or desktop applications in the way described.Option D is wrong because the syntactic requirements of XHTML are significantly stricter than those of HTML.

Common Pitfalls:
One pitfall is mixing HTML and XHTML syntax in the same document, which may confuse validators and browsers. Another issue is declaring a document as XHTML in the doctype but serving it with the wrong content type. Although modern HTML5 has simplified some of these distinctions, understanding the historical differences helps when maintaining older systems and reading legacy code that still uses XHTML style rules.

Final Answer:
XHTML is HTML rewritten as a stricter XML based markup language that requires well formed syntax and proper nesting.