DocBok is a set of tags that mark up logical structures of documents. It is not designed to control appearance of the documents.

         <para></para>
        

Suppose you thought you put a line feed doing the above. Now it may be displayed as you meant it to be but on the other style sheet it may use different color to recognize separate paragraphs and some may use sound even.

With the command above, it can only inform that an empty paragraph is existing there but nothing else.

And come to think of it, html is also similar in that regard, originally, that is.

Comment format <!-- ... --> seen often in html is not a comment tag which starts with <!-- and ends with -->.

A comment is a comment and inside a comment, -- should not be used as stated in the xml definition.

         <!--
         hogehoge
         fugafuga -- buroro
         -->
        

So a comment like above is not valid.

Seems it has become strange but originally here were sentences that were heavily influenced by the law "xml is not sgml" ...

In html, an empty element, e.g.,br tag must omit an end tag but an end tag can not be omitted in xml.

To rephrase it, in xhtml you must write br/ for br. It's the same story here.

In html you can omit "" to pass a value but in xml you can not omit it. Therefore,

         <sect1 id=hoge>
        

would be a mistake.

More likely, certain rules have been set for what can be omitted in html. A browser that can accept any kinds of syntax should be regarded as abnormal.

Xml distinguishes upper and lower case of characters in tag names.

         <sect1>
         ....
         </SECT1>
        

Above is incorrect as setting an end tag of SECT1 without a matching start tag.

Weeeell, say SECT1 as a tag is not defined anywhere in the script shown, so it is a mistake to begin with anyway.