EXTRA BITS: SGML HTML XML - Computerphile

TL;DR

XML is a proper subset within the SGML universe, while XHTML, mathematics markup, and SVG are applications built using XML notation. Published after about 18 months of drafting, XML required end tags and cleaned up case handling, giving browsers a reliable tree structure and front-end tools a clear target. Read on to understand why “subset” and “application” are crucially different terms.

Transcript

we've been wandering down the the buy roads of html x ml sgml all sorts of mls and various non-mls and last time i think we spoke some people had wandered off to the pub and sorted everything out over a pint so what happened about that then what happened next well after about 18 months the draft xml specification was published and everybody agreed ... Read More

Key Insights

• 🥺 XML specification addressed issues faced with HTML, including the requirement to include end tags, leading to a more structured and standardized approach to web development.
• 😚 XML enabled browser implementers to perform validation operations, ensuring that all tags were properly closed and the structure of web pages was accurate.
• 😫 XML served as the foundation for the development of cleaned up versions of existing tag sets, such as XHTML, and the creation of new tag sets, including mathematics markup and scalable vector graphics.
• 😫 Tim Berners-Lee's move to MIT and the establishment of the W3C played a significant role in setting web standards and promoting vendor neutrality.

Questions & Answers

Q: How are SGML, HTML, and XML related?

The speaker describes SGML and XML as enabling technologies or meta-syntaxes used to define tag sets. XML was created as a proper subset within the SGML universe, while HTML is presented as an application built on an enabling technology rather than merely another equivalent “ML.”

Q: What was the major difference between HTML and XML?

A major change was that XML did not permit omitted end tags. The draft specification also cleaned up about seven or eight other, more technical issues inherited from HTML.

Q: Why did XML require end tags?

Requiring every end tag allowed software to match opening and closing tags like matching curly braces in a compiler. A browser could therefore confirm that the document formed a tree even when it did not understand every tag.

Q: Why did browsers still need to tolerate missing end tags?

Some people continued to hand-code pages and might write repeated paragraph tags without closing them. Although tools such as Dreamweaver could aim to produce compliant pages, browsers still had to tolerate imperfect markup in practice.

Q: How did XML change uppercase and lowercase handling?

In SGML, uppercase and lowercase had been treated as equivalent, reflecting practices from the punch-card era. XML cleaned up that behavior so differently capitalized tag names would not all be treated as the same name.

Q: How did XML help browser and front-end software developers?

Browser implementers gained a dependable brackets-matching operation for checking whether tags formed a valid tree. XML also gave front-end software a clean target for generating consistently notated web pages.

Q: What is XHTML in relation to XML?

XHTML is described as cleaned-up HTML expressed in XML notation. Its tags match, and its structure is defined according to XML’s stricter rules.

Q: What tag sets were built using XML?

The transcript names XHTML, mathematics markup, and Scalable Vector Graphics, or SVG, as tag sets expressed using XML technology. It describes SVG as an application of XML and says thousands, perhaps tens of thousands, of tag sets were created on top of XML.

Summary & Key Takeaways

• The XML specification was published after 18 months, addressing the issues faced with HTML and introducing the requirement to include end tags in XML.

• XML enabled browsers to perform a brackets matching operation, ensuring that all tags were matched and validating the structure of web pages.

• XML served as the foundation for the development of cleaned up versions of HTML and the creation of numerous tag sets, including mathematics markup and scalable vector graphics.