HTML best practices
In this section, we'll provide some tips and best practices for your HTML documents.
Start with a template
This may seem like common knowledge, but creating and using a template for your HTML document can save you time and improve the development process. There's no sense re-inventing the wheel, so once you set up a structure for a basic web page, save it and reuse it. Your HTML template should include the key elements and tags that you typically find in a web page.
Here's an example of a typical HTML template for a BlackBerry WebWorks application:
<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0, user-scalable=no"> <title>Heading</title> <link href="favicon.ico" rel="icon" type="image/x-icon"> <link rel="stylesheet" href="css/app.css"> <style type="text/css"> </style> </head> <body> <h1>Heading</h1> <p>The quick brown fox jumps over the lazy dog.</p> <script src="js/app.js"></script> </body> </html>
In this template, there are a few things to note:
- The application is set to full screen by using the following values in the viewport meta tag: initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0
- Zooming functionality is turned off by setting user-scalable=no
Specify a document type
The DOCTYPE declaration tells the browser what version of HTML the page is written in, which informs the browser how to render the content. When you validate your web page, the HTML validator uses the DOCTYPE to determine which HTML version to validate against; the validator checks your code against the applicable standards and reports which portions are compliant and non-compliant.
Why should I specify it?
If you don't include a document type declaration, your content will not be rendered consistently across browsers. Each browser might render your content differently. By including a DOCTYPE, browsers are required to process your content according to the standards for that document type.
Which DOCTYPE should I use?
HTML5 is the current standard, which you should be using in your BlackBerry applications. HTML5 includes great new features and functionality that you can leverage in your application.
For BlackBerry devices running BlackBerry 6 and earlier, the browser doesn't support all of the HTML5 features, so you might choose to use the HTML 4.01 Strict or XHTML 1.0 Strict document types instead of the HTML5 document type.
Design your content for offline use
- Make your web application available offline
- Increase the speed of your application by caching resources locally
- Reduce load times and data usage by only downloading updated or changed resources
The application cache (also known as AppCache), allows you to specify the files to cache and make available even when the user is offline.
Create a manifest
To use the AppCache feature, your web application must provide a "manifest" to the browser that lists all the resources that it needs so that the application can function when offline. After the first time a web page is loaded, the resources specified in the manifest file are obtained from the application cache, not from the web server. The manifest is simply a text file that typically has a file extension of .appcache.
CACHE MANIFEST # 2012-03-08:v1 # Explicitly cached resources index.html css/app.css js/app.js # offline.html displays if the user is offline FALLBACK: / /offline.html # All other resources require an online connection NETWORK: * # Additional cached resources CACHE: img/logo.png
Your manifest file must be served with the mime-type text/cache-manifest. You may need to add a custom file type to your web server or configuration. For example, to serve this mime-type in Apache, add this line to your configuration file (httpd.conf):
AddType text/cache-manifest .appcache
To access the manifest file, you must declare it in your HTML document by adding the manifest attribute to the <html> tag. The manifest attribute points to the manifest file. If your web application has more than one page, make sure that each page includes the manifest attribute, otherwise those pages won't be part of AppCache, and they won't work offline.
Updating the appcache
- The user clears their cache.
- The manifest file is modified. Note: Updating the resources on your server will not trigger a cache update. You must change the manifest file itself.
- The application cache is programmatically updated.
Use the correct viewport syntax
The viewport is the rectangular area that the user sees on their mobile device. You can override the default viewport by using the viewport <meta> tag in the <head> section of your HTML document.
Keep in mind that the syntax for the viewport meta tag is not the same as the syntax for the @viewport rule in CSS. This means that you must use commas as delimiters between viewport values, and not semicolons. If you use semicolon delimiters, the viewport values can be interpreted incorrectly.
Here's an example showing the correct syntax for the viewport meta tag:
<meta name="viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0, user-scalable=no">
Create a site icon
Every web browser looks for a favicon.ico (short for favorites icon) file and displays the file as the site's icon. You often see the small icon displayed in the address bar, tabs, user's bookmarks or favorites.
When the browser looks for your web page, it will also make a request for your favicon.ico file. If the favicon.ico file can't be found, you'll see 404 errors in your web server's log. To avoid this, you should always specify a favicon.ico file in your web pages.
[Mon Aug 20 15:17:49 2012] [error] [client 192.0.2.2] File does not exist: C:/www/favicon.ico
In the <head> section of your HTML5 documents, add a link tag pointing to your favicon.ico file.
<link href="favicon.ico" rel="icon" type="image/x-icon">
Use external files
For BlackBerry WebWorks applications, the resources are typically bundled with the application. The following information applies mainly to web (browser-based) applications.
The benefits for external files aren't experienced in every situation. Before you start externalizing your application's resources, consider only making the resource external if:
- The resource is used in more than one HTML document
- The resource changes infrequently
Here are some tips when using external files in your web application:
- Source your files off a well-known host, which can reduce the number of DNS lookups.
- Spread the resources out to another domain. The browser has a limited number of connections that it will open to a particular domain; if you spread the files out to another domain, it increases the browser’s freedom to parallelize requests.
(for example, jQuery). This can
improve your web application's performance by decreasing latency and improving
what's cached. For example, if a user has visited another site that uses CDN hosted
files, those files will already be cached on the device. When the user visits your
site for the first time, the library will already be available. Here's a code
example that uses the Google's CDN-hosted
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"> </script>
- Analyze the performance of your web application by using the Web Inspector. For more information about the Web Inspector, see Debugging using Web Inspector.
Put your stylesheets at the top
Another way to improve the loading speed of your page is to place your stylesheets at the top of your HTML document. When you have stylesheets in the <head> section of your document, the page renders progressively so it appears to load faster and provides visual feedback to the user as they wait. If your stylesheets are near the bottom of the document, the browser blocks progressive rendering to avoid having to redraw elements of the page if their styles change, so the user ends up with a blank white page. Having stylesheets at the top also increases parallelization because, unlike scripts, stylesheets do not require synchronous processing.
<!DOCTYPE html> <html> <head> ... <link rel="stylesheet" href="styles1.css"> </head> <body> ... </body> </html>
Do not import stylesheets
When adding external stylesheets to your HTML document, do not use the @import rule. You should use the <link> tag instead.
If you use the @import rule to import style sheets, in some browsers, it behaves as though your stylesheet is at the bottom of the HTML document. This is not the behavior you want because because it prevents the web page from loading progressively.
Not recommended: Using @import
<style> @import url("styles2.css"); </style>
Recommended: Using a LINK tag
<link rel="stylesheet" href="styles1.css">
In addition, the @import rule can be nested inside CSS files, preventing the browser from loading your stylesheets in parallel. For example, if styles1.css contained the following:
The browser must first retrieve styles1.css and then retrieve styles2.css.
Put scripts at the bottom
Putting your script at the bottom also improves parallel downloading components in parallel. While a script is downloading, parallel downloading is disabled. If the script is at the top, the browser must wait for the script to download before downloading any other components.
Unless you're using the deprecated document.write to generate the body, your scripts can typically be moved to the bottom of the HTML document.
Another advantage to putting the script in the BODY (before the closing body tag) is that you don't need to check to see if the DOM is loaded, because everything in the DOM has already loaded.
Recommended: Placing the script near the bottom of the BODY
<html lang="en"> <body> ... <script src="js/app.js"></script> </body> </html>
Not recommended: Placing the script in the HEAD
<html lang="en"> <head> <script src="js/app.js"></script> </head> <body onload="init();"> ... </body> </html>
Some third-party frameworks require that you include the scripts at the very top of the HTML document. Refer to the third-party's documentation for more information.
Defer script loading
<script src="js/app.js" defer></script>
Unfortunately, the DEFER attribute behaves differently between browsers. To avoid cross-browser issues, you can use the "lazy loading" approach. That is, avoid loading scripts until they are needed.
Here's an example of an unobtrusive lazy load pattern:
Last modified: 2013-11-27