The 5 Most Common Mistakes HTML5 Developers Make: A Beginner’s Guide

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It’s been over 20 years since Tim Berners-Lee and Robert Cailliau specified HTML, which became the standard markup language used to build the Internet. Ever since then, the HTML development community has begged for improvements to this language, but this cry was mostly heard by web browser developers who tried to ease the HTML issues of their colleagues. Unfortunately, this led to even more problems causing many cross-browser compatibility issues and duplication of development work. Over these 20 years, HTML was upgraded 4 times, while most of the browsers got double-digit numbers of major updates plus numerous small patches.

HTML5 was supposed to finally solve our problems and become one standard to rule them all (browsers). This was probably one of the most anticipated technologies since creation of the World Wide Web. Has it happened? Did we finally get one markup language that will be fully cross-browser compatible and will work on all desktop and mobile platforms giving us all of those features we were asking for? I dont know! Just few days ago (Sept. 16th 2014) we received one more call for review by W3C so the HTML5 specification is still incomplete.

Despite HTML5 issues and mistakes, is it the one language ring to rule them all?

Hopefully, when the specification is one day finalized, browsers will not already have significant obsolete code, and they will easily and properly implement great features like Web Workers, Multiple synchronized audio and video elements, and other HTML5 components that we’ve needed for a long time.

Give hasty developers an incomplete spec, and you'll have a recipe for disaster.

We do however have thousands of companies that have based their businesses on HTML5 and are doing great. There are also many great HTML5-based applications and games used by millions of people, so success is obviously possible and HTML5 is, and will continue to be, used regardless on the status of its specification.

However, the recipe I mentioned can easily blow up in our faces, and thus I’ve emphasized some of the most basic HTML5 mistakes that can be avoided. Most of the mistakes listed below are consequence of incomplete or missing implementation of certain HTML5 elements in different browsers, and we should hope that in the near future they will become obsolete. Until this happens, I suggest reading the list and having it in mind when building your next HTML5 application whether you’re an HTML5 beginner or an experienced vet.

Common mistake #1: Trusting local storage

Let them eat cake! This approach has been a burden on developers for ages. Due to reasonably sensible fear of security breaches and protection of computers, in the “dark ages” when the Internet was feared by many, web applications were allowed to leave unreasonably small amounts of data on computers. True, there were things like user data that “great browser masters from Microsoft(r)” gave us, or things like Local Shared Objects in Flash, but this was far from perfect.

It is therefore reasonable that one of the first basic HTML5 features adopted by developers was Web Storage. However, be alert that Web Storage is not secure. It is better than using cookies because it will not be transmitted over the wire, but it is not encrypted. You should definitely never store security tokens there. Your security policy should never rely on data stored in Web Storage as a malicious user can easily modify her localStorage and sessionStorage values at any time.

To get more insight on Web Storage and how to use it, I suggest reading this post.

Common mistake #2: Expecting compatibility among browsers

HTML5 is much more than a simple markup language. It has matured enough to combine behavior together with layout, and you should consider HTML5 as extended HTML with advanced JavaScript on top. If you look at all the trouble we had before just to make a static combination of HTML+CSS look identical on all browsers, it is fair to assume that more complexity will just mean more effort assuring cross-browser compatibility.

HTML5 interpretation on different browsers is far from identical, just like the case was with JavaScript. All major players in the browser wars lended a hand in the HTML5 spec, so it’s fair to assume that deviations between browsers should reduce over time. But even now some browsers decided to fully ignore certain HTML5 elements making it very difficult to follow a baseline and common set of features. If we add more internet connected devices and platforms to the equation, the situation gets even more complicated, causing problems with HTML5.

For example Web Animations are great feature that is supported only by Chrome and Opera, while Web Notification feature that allows alerting the user outside the context of a web page of an occurrence, such as the delivery of email, is fully ignored by Internet Explorer.

To learn more about HTML5 features and support by different browsers check out the HTML5 guide at

So the fact remains that even though HTML5 is new and well specified, we should expect a great deal of cross-browser compatibility issues and plan for them in advance. There are just too many gaps that browsers need to fill in, and it is fair to expect that they cannot overcome all of the differences between platforms well.

Common mistake #3: Assuming high performance

Regardless of the fact that HTML5 is still evolving, it is a very powerful technology that has many advantages over alternate platforms used before its existence. But, with great power comes great responsibility, especially for HTML5 beginners. HTML5 has been adopted by all major browsers on desktop and mobile platforms. Having this in mind, many development teams pick HTML5 as their preferred platform, hoping that their applications will run equally on all browsers. However, assuming sensible performance on both desktop and mobile platforms just because HTML5 specification says so, is not sensible. Lots of companies (khm! Facebook khm!) placed their bets on HTML5 for their mobile platform and suffered from HTML5 not working out as they planned.

Again, however, there are some great companies that rely heavily on HTML5. Just look at the numerous online game development studios that are doing amazing stuff while pushing HTML5 and browsers to their limits. Obviously, as long as you are aware of the performance issues and are working around these, you can be in a great place making amazing applications.

Common mistake #4: Limited accessibility

Web has become a very important part of our lives. Making applications accessible to people who rely upon assistive technology is important topic that is often put aside in software development. HTML5 tries to overcome this by implementing some of the advanced accessibility features. More than a few developers accepted this to be sufficient and haven’t really spent any time implementing additional accessibility options in their applications. Unfortunately, at this stage HTML5 has issues that prevent it from making your applications available to everyone and you should expect to invest additional time if you want to include a broader range of users.

You can check this place for more information about accessibility in HTML5 and the current state of the most common accessibility features.

Common mistake #5: Not using HTML5 enhancements

HTML5 has extended the standard HTML/XHTML set of tags significantly. In addition to new tags, we got quite a few new rules and behaviors. Too many developers picked up just a few enhancements and have skipped some of the very cool new features of HTML5.

One of the coolest things in HTML5 is client-side validation. This feature was probably one of the earliest elements of HTML5 that web browsers picked up. But, unfortunately, you can find more than a few developers who add novalidate attribute to their forms by default. The reasons for doing this are reasonable, and I’m quite sure we will have a debate about this one. Due to backward compatibility, many applications implemented custom JavaScript validators and having out-of-the-box validation done by browsers on top of that is inconvenient. However, it is not too difficult to assure that two validation methods will not collide, and standardizing user validation will ensure common experience while helping to resolve accessibility issues that I mentioned earlier.

Another great feature is related to the way user input is handled in HTML5. Before HTML5 came, we had to keep all of our form fields contained inside the <form></form> tag. New form attributes make it perfectly valid to do something like this:

<form action="demo_form.asp" id="form1">
  First name: <input type="text" name="fname"><br>
  <input type="submit" value="Submit">

Last name: <input type="text" name="lname" form="form1">

Even if lname is not inside the form, it will be posted together with fname.

For more information about new form attributes and enhancements, you can check the Mozilla Developer Network.

Wrap up

I understand that web developers are collateral damage in the browser wars as many of the above mistakes are a direct consequence of problematic HTML5 implementation in different browsers. However, it is still crucial that we avoid common HTML5 issues and spend some time understanding the new features of HTML5. Once we have it all under control, our applications will utilize great new enhancements and take the web to the next level.

Web developers are collateral damage in the browser wars.

About the author

Demir Selmanovic, Bosnia and Herzegovina
member since May 23, 2014
Demir is a developer and project manager with over 15 years of professional experience in a wide range of software development roles. He excels as a solo developer, team member, team leader, or manager of multiple distributed teams. He works closely with clients to define ideas and deliver products. [click to continue...]
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Ron Barak
"which became the standard markup language used to build the Internet." is actually wrong. You may say "which became the standard markup language used to build the WWW." but if you're looking for a 'language' "language used to build the Internet." you'd want to choose the TCP/IP protocols.
Svetlana Linuxenko
Haha, don't argue with habits, it's all just about XML ))
If I m using HTML 5 in my project. Is it good or bad
Svetlana Linuxenko
Idk why do you asking me about that, but ill answer in the key of the discussion - you have no choice, html5 not entirely about your doctype, there is a lot of things browsers support : prefetch &lookup, location, storage etc..
bcz i m creaitng a college project in html5 and css3 , anguler and express so that is why i asked about browsers support......... but thanx
Bruno Seixas
HTML5 takes on very situations that needed attention and with time I assume all it´s "addon´s" will sink in and issues like backward compatibility will be a laugh of the past. Maybe a very happy vision but one can hope :)
Sylvain Pollet
Please do not make Facebook an example about HTML5 performance. Facebook HTML5 app was developed by an incompetent team, with many flaws spotted by catching the data exchange. Some have shown that this kind of HTML5 application could be almost as fast as native solutions:
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About the author
Demir Selmanovic
JavaScript Developer
Demir is a developer and project manager with over 15 years of professional experience in a wide range of software development roles. He excels as a solo developer, team member, team leader, or manager of multiple distributed teams. He works closely with clients to define ideas and deliver products.