Mozilla, as an organization and product, has made implementing and developing open standards a core part of its mission from the beginning. But what is so important about standards?
I was listening to a presentation by Mitchell the other day, and one phrase in particular stood out: “Standards compliance is important because it ensures the portability of my data”. We don’t expend huge amounts of time and effort on open standards and standards compliance because of marketplace pressure. Open standards are a guarantee that the documents and data that our users are reading and writing today will continue to be available to us permanently in the future. Open standards are an essential service to the users of Firefox and to the web as a whole.
This is why Mozilla has spent so much effort implementing open video and the <video> element in HTML. And this is why open video is fundamentally better for users than Flash video or even patent-encumbered formats such as H.264.
There are many different kinds of user agents for HTML markup. The most popular and most important user agents today are display-based web browsers. There are other types of HTML consumers, such as search engines, aggregators, text-based browsers, and speech browsers. There are also many different types of producers of HTML.
One of the the most important features of the HTML5 specification from the perspective of user agents is that it specifies how to parse and consume all markup, not just correct markup. The vast majority of markup on the web is not “valid”. This will undoubtedly continue to be true, and it’s not a bad thing: imagine a dystopian world where only complex tools or skilled technicians could create web content!
The HTML5 parsing specification contains rules to transform any possible sequence of characters or bytes into a standard document object model. From conversations with Ian, I believe this was one of his primary goals for the initial HTML5 specification. I’m a little surprised that this is not called out more clearly in the parsing section of the specification.
Setting aside the unanswerable questions of whether generic metadata can be used to solve problems at web-scale, or whether RDFa can solve the metadata problem, most of the discussion on the WhatWG mailing list regarding whether RDF (RDFa) should be integrated into the HTML specification has focused on whether the RDFa would make the markup invalid HTML.
If everyone actually implements the HTML5 parsing specification, who cares whether it’s valid markup? You get the same document structure in every case. User agents which are aware of RDFa and wish to use it to solve problems may do so. This seems like the ultimate extensibility mechanism you could possibly want. The only “problem” is that a validator (conformance checker) will warn you that you’ve produced invalid markup.
Perhaps, rather than ranting about invalid markup, the specification should be altered. Remove all references to parse errors, and instead require parsers to interoperably transform any possible sequence of characters into the same document model. Then those who wish to use RDFa markup may do so, and user agents can ignore or process this metadata as appropriate.
One possible objection to this error-less regime is that it doesn’t give useful guidance to content authors or authoring tools. If everything is valid HTML markup, there still should be best practices for authoring HTML. If you want your content to work with existing browsers, reverse-engineering existing practice is a tricky exercise. Perhaps there should be a section of the HTML5 specification indicating how authoring tools should interoperably serialize a given document model. Then it would be relatively simple to write a fuzz tester to take a document, serialize it per the serialization spec, re-parse it per the parsing spec, and then compare the results for equality.
The discussions about HTML and RDF have drifted from practical interoperability into theoretical “validity”, purity, and architectural grandiosity. Let’s get back to the specific technical question that needs to be answered: if you embed RDFa in HTML5, does it parse into a usable DOM? If not, are there specific changes to the parsing specification that will allow it to parse to a usable DOM?
If you are able to ignore Internet Explorer, all the other major browser can render SVG content. Recently, while writing previous posts and webapps, I discovered several quirks that may cause browsers to fail to render SVG content consistently.
Inline SVG is an attractive option because it doesn’t require an external file. Unfortunately, inline SVG has one significant problem: authors are forced to use XHTML and the application/xhtml+xml MIME type, instead of standard HTML and the text/html MIME type. XHTML is not a very forgiving language, and one I would generally discourage. Depending on authoring tools and other content on the page, producing valid XML may be difficult. Even more problematic is that Internet Explorer will refuse to display the page at all; there is no graceful fallback for the majority browser. Finally, syndicating SVG in feeds will often cause the SVG to be stripped as it is syndicated.
I used inline SVG for my Mozilla compiler warnings front end, because I don’t care about Internet Explorer users in that application. But it has very limited usefulness in general.
Referencing external SVG content
At first glance, it might seem that you could reference an SVG document using the HTML <:img> element., but this is not the case. SVG images are complete sub-documents. They have their own script context and can script themselves. They can also load additional sub-documents such as images. Because of this, browsers force authors to embed SVG images using <object> or <iframe>.
Embedding SVG with <object>
The <object> element is the generic HTML mechanism for embedding external content. It can be used just like an <iframe> for external HTML document. It can be used to embed plugin-rendered content such as Flash, and it can be used to embed SVG:
Alternate markup here. If you see this, your browser may not support SVG, or a content aggregator may have stripped the object element.
The object element is the best choice in most situations. All browsers including Internet Explorer will display the fallback content if they don’t know how to display SVG or if the image won’t load. Using the object element, authors can even pass parameters to the SVG document.
Embedding SVG with <iframe>
It is also possible to include SVG content using the <iframe> element.
<iframe width="350" height="250"
Alternate markup here. If you see this, your browser might not support iframes, or a content aggregator might have stripped the iframe element.
There are minor but important differences using iframe rather than object to display SVG: Internet explorer will load the iframe but choke on the SVG content. The user won’t skip back to the fallback content within the <iframe>element, and in some cases the user may see a download prompt for the SVG document. But many content sanitizers such as those found in feed aggregators will allow <iframe> through while rejecting <object> And finally, iframes have a border by default. You can remove this border using CSS.
Use this MIME type: image/svg+xml
The correct MIME type for SVG content is image/svg+xml. Firefox will accept application/svg+xml but Safari will not!
Specify image dimensions
The author should know the image dimensions in advance. If you don’t specify the width and height in the <object> or <iframe> element, browsers will initially size the object at 300×150 pixels, and then their behavior will diverge:
Resize to the image intrinsic size, once loaded
Scroll overflow content
Crop overflow content
Don’t use rgba() colors in SVG
The CSS3 specification allows for any color to be specified with transparency using rgba syntax. Many web browsers support RGBA colors for HTML content, but only Firefox supports them for SVG content. Instead of using rgba colors, use the SVG properties fill-opacity and stroke-opacity for maximum portability.