This is the exerpt of my Opera Mini review for Mail Ru where we had to decide what browsers we had to support for one of our mobile projects and what functionality had to be provided for the target platforms.
Every request to a website is done not directly from the client installed on the phone, but through Opera servers farm in Norway, Poland, Korea, China and the United States as per this press release.
Opera Mini is just a viewer (with a few features) for the website rendered image that is composed on the server by Presto rendering engine (and now with Presto’s death all the servers may eventually be upgraded to Webkit). This image is rendered in a proprietary format called OBML (Opera Binary Markup Language). It has “language” word because though being compressed, it presents some dynamic features.
So Opera Mini is basically a very thin client that doesn’t have its own HTML/JS/CSS engines and hence the following consequences apply:
Opera Mini cannot do any recalculations on its own – no animation, no background processing, no timers, no generated events.
as the actual page is rendered somewhere on the server possibly in another country, you won’t be able to access your localhost or your protected network from neither Opera Mini emulator nor from a real device. The address you are trying to fetch must be publicly accessible from the outside world.
There is nothing to debug on the actual device and there’s no way Opera will provide remote debuggers access to their servers. Hence ther are no development toolbars and no script debugging. You’re back to alerting debugging information, and even with alerts Opera Mini doesn’t make it easy for you – usually only one alert at a page load is displayed, even if you put few alerts calls one after another in the code.
Opera mini has two rendering modes: desktop view and mobile view. Desktop view is enabled by default and mobile view can be enabled in the settings.
Desktop view makes Opera Mini report its media type to CSS as screen and the corresponding media queries fire.
Desktop mode has two viewing modes:
However, if the first page the user visits is very wide, “zoomed out” mode will be enabled for this page regardless of the samePage property value, and only the next page will respect the corresponding value.
In the desktop view mode page content is not reflowed.
In the mobile view mode page content is reflowed into one column to fit in the screen width.
Opera Mini starts reporting its Media as handheld and corresponding media queries are applied in the CSS.
There’s no way to prevent user from changing rendering modes and the reflow rules Opera Mini uses are not published or documented, so targeting mobile view is very problematic and can only be experimentally driven.
Currently Opera servers use Presto (though the desktop version of Opera abandoned Presto in favour of Webkit).
The following features are supported:
And the following features are not supported:
Active area state is considered to be changed when one of the elements contained within this active area is clicked or form values are changed, neither scrolling nor other actions cause Opera Mini to request fresh page state from the server.
The following events are fired on the server:
Opera Mini provides additional information about the phone it runs on. Opera Mini somehow sends this information to its servers and then the server sends some information to the requested URL via HTTP headers. Note that OBML is binary and not documented, so we don’t know what exactly is sent to its servers in Norway or elsewhere, we can only observe what these servers request from our servers. Also note that true privacy can not be ensured because all the communication is done through their servers and noone knows what statistics or data they collect.
Opera Mini servers have the following headers when requesting data from target servers:
User-Agent: Opera/9.80 ($PLATFORM_NAME$; $PRODUCT_NAME$/$CLIENT_VERSION$/ $SERVER_VERSION$;U; $LOCALE$) $PRESTO_VERSION$ $EQUIV_DESKTOP_VERSION$
X-OperaMini-Features: Basic comma-separated list of features supported on the device.
advanced - MIDP2-version (has more RAM)
basic - MIDP1-version (usually installed on the cheapest phones)
camera - shows support for photos file upload using HTML input type=file. Unfortunately there’s no available statistical data (or a list of models) on what phones support this and what don’t.
file_system - file system support (i.e. user is able to download and/or upload files)
folding - content folding feature status
secure - connection between Opera Mini and Opera Mini servers is encrypted.
touch - Opera Mini is running on a touch-operated device
X-OperaMini-Phone: phone model and vendor, useful for the project audience statistics.
X-Forwarded-For: the list of proxy agents between the Opera Mini servers and your server, the first entry in the list shows the actual phone connection IP address. This is the only place where the actual phone’s IP address is shown.
window.operamini.page.maxAge - time that the client will keep the page cached (minutes)
window.operamini.page.samePage - indicates if the pages is forced be left in zoomed in mode or it can be allowed to be switched to zoomed out mode
window.operamini.sms - shows sms send dialogue as soon as window.operamini.sms.number and window.operamini.sms.body values are filled in. This method could be used to ask user to send some sms somewhere (even paid ones). The availability of sms sending feature can be checked by window.operamini.features.sms
window.operamini.remote - X-Forwarded-For analogue, first value in the list is the client IP address, other values are just a list of proxies between Opera Mini server and your server. Most of the times, the list only contains one value - phone IP address.
There are some hidden settings available in Opera Mini client. To toggle any of them, type the corresponding name into the address bar and press “Go”.
config: - most of the additiional settings
server:source - shows the html page source (in the exact form as it was sent to Opera Mini servers by your server for the requested URL). Unfortunately, the source is rendered in the ugly monospace font (so ugly that it’s obvious it’s included in Opera Mini package). As OBML format is binary and not documented, inspecting this source is the only view-source option a client-side developer has.
opera:cache - shows the elements cached by the client
about: - shows all versions and patent information.
:before selectors with
content property on form elements such as inputs – input value can be erraneously overwritten by the
content property value. I witnessed this on a live project - input value was overwritten right before form submit.