Jeremy Reimer Revies the new beta version of th opera browser in his new article posted on the Ars Technica magazine site. He writes:

«Opera has consistently defied expectations about what a browser (and browser company) can accomplish. Begun back in 1994 as a research project at the Norwegian research company Telenor, the Opera browser was first released as version 2.0 in 1996, and ported to multiple platforms in 1998. Originally a paid-for browser, Opera became ad-supported with version 5, but the ads were removed in the 8.5 release, bringing the browser to completely free status. Version 9 offered many improvements, and now the company has released a second beta preview of version 9.5, which Ars first took a look at last year, back when it was in alpha. »

Source: Ars Technica.

Lars Kleinscmidt published an intresting article about skinning Opera broser. Here's teh short list of topics that discussed in this article:

• Locating the skin files

• Unpacking / packing skins

• Standard skin folder/file list

• Example 1: replacing a toolbar button

• Example 2: adding a custom toolbar button

• Example 3: adding a background image to Speed Dial

Source: Opera Developers Community.

There is a intresting tutorial about creating Opera widgets on Opera Developer Community site. Widgets are small web applications running on your desktop. They are implemented using client-side web technologies, and creating one is very much like creating a web page, except that it is run in a slightly different context. This tutorial contain all basic infoemation and will guide you through all steps of creating the Opera widget. You can read the full version of this tutorial on Opera Developer Community site.

The Opera Community continues posting a series of article about features of  new HTML5.  In this time you can read about canvas - the new addition of HTML5 and new 3d context.
There is a lot of examples also
Source: Tim's Blog

Introduction

Opera Mini is a very clever way of bringing the web to your mobile phone - it will work on most phone models, even low spec ones, as long as they will run a JVM. Basically, when you request a web page from Opera Mini, a request is sent to the Opera Mini servers. They retrieve the page, convert it into OBML (Opera Binary Markup Language,) a very compact binary markup format that reduces the page size by up to 90%, and then serve it to your phone.

This tutorial is intended for webmasters or users with a technical interest and some experience with HTML and/or JavaScript. It will demonstrate relatively simple ways of getting to the root of the problem if a JavaScript does not work as expected in Opera.

Read it here

Using the Opera Developer Console

The Opera Developer Console is a web technology-based tool for inspecting and analyzing various aspects of a webpage during development or debugging. The Developer Console exposes various aspects of the DOM, the JavaScript enviroment and the CSS attached to the document. This document is a quick introduction to the Developer Console, and users are expected to be familiar with the DOM and CSS. Some knowledge of JavaScript is also an advantage, but not a prerequisite.

Opening the Developer Console

Opening the developer console is achieved by going to the Advanced submenu of the Tools menu, and selecting the Developer Console option. When you do this, Opera will open a Developer console instance for the currently active document.

Console basics

When the Developer Console is opened, the different tools are made available in one of five different tabs:

The basis of a Web application is usually HTML. The HTML documents are translated into a DOM data structure by the browser. The DOM tree can be modified through scripting, often in ECMA/JavaScript, where elements can be added, changed, or removed through DOM bindings in the scripting environment. The result is a document that can change dynamically. Combine this with networking capability through the XMLHttpRequest object, and you have a fully working Web application. Opera also has support for various multimedia such as SVG, Canvas and the Audio object.

HTML - HyperText Markup Language

HTML is the oldest and most widespread language on the Web. It describes hyperlinked documents where the content is marked up with tags, or elements, that identify semantic parts of the document, such as paragraphs, headings, and lists.

<!DOCTYPE html>
<html>
  <head>
    <title>A Web page</title>
  </head>
  <body>
    <h1>A Web page</h1>
    <p>A <a href="foo.html">link</a>.</p>
  </body>
</html>
 

HTML has several types of elements divided into two categories, block elements and inline elements. Block elements separate the document into different types of sections. Examples include the p (paragraph), ul (unordered list) and table (tabular data) elements. Inline elements refer to parts of a sentence, for example surrounding a set of words. Examples include the q (quote), code (program code) and dfn (definition) elements. Through the use of these different elements, content can be marked up to identify the different kinds of content in the document. The different elements can have attributes, name-value pairs which further qualify the elements with information. Examples include the id (identifier), href (hypertext reference, a link URL), and lang (language) attributes.

HTML comes in two flavours: HTML and XHTML. The latter is an application of XML, meaning that it is required to be well formed, but thus also easier to parse. The content of the languages is mostly the same, meaning the same elements and attributes are available. However, there are slight differences in how Opera handles the two. See the document on doctypes in Opera [OPERADOCTYPE] for more information.

See the HTML specification [HTML4] and the XHTML specification [XHTML] for more information.

XML - Extensible Markup Language