Making Your Pyjamas App HTML5-compliant

I recently decided to update my pyjamas app so that it’s HTML5-compliant. I did this because I recently customized my pyjamas version to work in Internet Explorer 9 (I needed true IE9 support, not the backwards compatibility mode current version of pyjamas provides). Unfortunately, IE9 seems to be very anal about it’s rendering modes, it will either render the page in strict HTML5, or revert to half-broken ‘Quirks Mode’ that doesn’t even support event handler. It seemed easier to tweak my app to be compatible with HTML5 (besides I’d have to do so eventually anyway) than to try to write a special version of event handler for IE just so that I can run my app in a mode that’s not even meant to be used by modern websites anyway.

First of all, I had to tell the browser to render the page in HTML5 mode. To do so, I opened the main .html file for the app (in project’s public directory) and added the DOCTYPE at the very top (before <html> tag):

<!DOCTYPE html>

This tells the browser to render the page using the latest html standard (which happens to be HTML5). Second, I noticed that my fancy “loading” gif no longer appeared in the center of the page (not vertically at least). Apparently, setting my pyjamas frame to 100% height was no longer good enough. In HTML5, I now have to set both, <body> and <html> tags to take up 100% height:

html, body {

The next issue with HTML5 that I noticed is that all my images now seemed to have a small footer below them (a few pixels of space below the image). This is probably not a big deal for most, but when you use images as icons, or need to fit exactly within a certain container, it can definitely mess up your layout. The reason for that is that HTML5 modified default display style for images to ‘inline’, which treats them the same way as text, giving them a ‘line-height’ property. There are two ways to fix the images back to normal. One way is to define ‘line-height’ for the image as 0:

.gwt-Image {
    line-height: 0;

This was my initial solution, but there is a problem with it. There is a bug in Firefox that overrides line-height, even if you specify ‘!important’ after it. The other alternative is to change image back to the way it was rendered before HTML5:

.gwt-Image {
    display: block;

This solution works in all browsers. Additionally, I noticed that the the text I placed inside tables using pyjamas’ Label() class rendered with too much spacing, as if it put every cell inside

tags. Turns out the work around for that was the same as for the images (even though the problems differ). To render the tables correctly, I modified the above code as follows:

.gwt-Table > tbody, .gwt-Image {
    display: block; /* HTML5 effects panel alignment fix, and footer removal fix for images */

Note that for the above fixes to work, you must not be overriding the .gwt-Image that pyjamas assigns to images, and should add .gwt-Table style to tables:


Finally, I noticed that my fonts were no longer consistent and that tables whose dimensions were defined via CSS were rendering incorrectly as well. The reason for that is that HTML5 no longer assumes that unitless numbers correspond to pixels. Which means that for all fonts, you now need to define “pt”, “px”, or “em” after the number, unless the number is zero. So something like

.text {
    font-size: 10;

should be changed to

.text {
    font-size: 10px;

The same change also applies to tables, divs, etc. If you have something as follows in your CSS:

.some-box {
    width: 200;
    height: 150;

you should change it to

.some-box {
    width: 200px;
    height: 150px;

I realize that some web designer is probably reading this now laughing at me, since I should have known to define the units to begin with. However, coming from a programming background, I see CSS more as a necessary evil, and usually stop tweaking it as soon as the page renders as expected (and up until HTML5, it rendered as expected without ‘px’ defined). Along the same lines, if you use Widget.setSize(), Widget.setWidth(), or Widget.setHeight() methods in pyjamas, make sure that you pass in the units as well (Widget.setSize('100px', '50px')) or use Widget.setPixelSize() instead. That’s all you need to do to make your pyjamas app HTML5-compliant.

To summarize:

  1. Add <!DOCTYPE html> tag at the top of the main .html file
  2. Set html and body to 100% height in CSS
  3. set images and table’s tbody to display: block
  4. Add units to every number in CSS (unless the number is 0)

There are a few IE9-specific quirks that this will not solve, but I’ll discuss them in my next post.

This entry was posted in Frameworks, Hacks and Tricks and tagged , , by Alexander Tsepkov. Bookmark the permalink.

About Alexander Tsepkov

Founder and CEO of Pyjeon. He started out with C++, but switched to Python as his main programming language due to its clean syntax and productivity. He often uses other languages for his work as well, such as JavaScript, Perl, and RapydScript. His posts tend to cover user experience, design considerations, languages, web development, Linux environment, as well as challenges of running a start-up.

One thought on “Making Your Pyjamas App HTML5-compliant

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>