Official Qt has depreciated QtWebkit. The last few releases are in not really supported "community releases".
A group of developers has started a new fork of Qt based webkit. This fork might be adopted within Qt, but no indication yet when or if.
https://github.com/annulen/webkit/wiki

A trial is started now in kde-next only to fully replace qt5-webkit with qtwebkit-tp. It has been tested locally for a few days already.
Otter-browser is being developed specifically for this fork. Reason to add to the repo is to try and see if there is a viable non-blink/chromium & non-gtk, lightweight browser available. No secret that both firefox or chromium based are not light on the system resources.

To make otter-browser integrate, go to about:config > interface and set as in example below:
otter

Problem to update

    $ upd
 
Checking kde-next build core main apps...
 
[kde-next] is synced.
[build] is synced.
[core] is synced.
[main] is synced.
[apps] is synced.
 
:: Synchronizing the packages' databases…
 kde-next is updated
 build is updated
 core is updated
 main is updated
 apps is updated
:: Starting full system update…
:: Replace qt5-webkit with kde-next/qtwebkit-tp?  [Y/n]  ( I answered yes )
Resolving dependencies…
Looking for inter-conlicts...

Package (6)             Old version  New versión  Net difference

kde-next/libksysguard   5.8.90-2         5.8.90-3              0,00 MiB
kde-next/liri-browser   0.3.15-1         0.3.16-1              0,01 MiB
kde-next/otter-browser  0.9.10.91-1      0.9.12-1              0,63 MiB
qt5-webkit              5.8.0beta-1                          -35,97 MiB
kde-next/qtwebkit-tp                     5.6.2-1              41,62 MiB
build/sqlite            3.16.0-1         3.16.1-1              0,00 MiB

Total download size:  55,42 MiB
Total installed size:     6,29 MiB

:: Proceed with instalation? [Y/n]    ( I answered yes )
(5/5) Checking the deposit keys                                                           
[-----------------------------------------------------------] 100%
(5/5) checking packages integrity                                                     
[-----------------------------------------------------------] 100%
(5/5) cargando los archivos de los paquetes                                                          [-----------------------------------------------------------] 100%
(5/5) checking for file conflicts
[-----------------------------------------------------------] 100%
Error: failed operation (conflicting files)
qtwebkit-tp: /usr/include/QtWebKit/QWebDatabase exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebElement exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebElementCollection exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebFullScreenVideoHandler exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebHapticFeedbackPlayer exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebHistory exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebHistoryInterface exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebHistoryItem exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebKitPlatformPlugin exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebNotificationData exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebNotificationPresenter exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebPluginFactory exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebSecurityOrigin exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebSelectData exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebSelectMethod exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebSettings exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebSpellChecker exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QWebTouchModifier exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/QtWebKit exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/qwebdatabase.h exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/qwebelement.h exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/qwebhistory.h exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/qwebhistoryinterface.h exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/qwebkitglobal.h exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/qwebkitplatformplugin.h exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/qwebpluginfactory.h exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/qwebsecurityorigin.h exists in the file system
qtwebkit-tp: /usr/include/QtWebKit/qwebsettings.h exists in the file system
Errors occurred, so the packages were not updated

Please use code tags when posting cli output, done for you this time.

My Spanish is not good enough to see what you did in this update and what you answered when asked to remove qt5-webkit when asked to.
And show what owns the conflicting files, example:

pacman -Qo /usr/include/QtWebKit/QWebDatabase

I edited my post in English and I wrote my answers.

The output of the command "pacman -Qo" is always the same:

Is contained in qtwebkit 2.3.4-2

When on a rolling distro you have to take care of your install and regularly remove obsolete packages. All of Qt4 has left a long time ago.
You really need to make a habit of running pacman -Qm once a month or at least once a quarter and remove packages no longer maintained/in the repos.

It is true. I forgot to remove clipgrab.