I use the below bookmarklet to help maintain my list on MyAnimeList.net (I use it in FireFox so I don't know if it works elsewhere.)
I periodically use this to add information to anime that I have added to my list since the last time I ran it.
Cover CSS Generator and Tag Updater
It generates one line of CSS per anime to add covers to your list based on a template. It will not re-process an anime that already has a style (existing styles must manually be copied to the top textarea). It will remove styles for anime no longer on your list, and add missing ones.
It can add an anime's English title (or first synonym), season, year, studios, and genres to the end of the Tags. It checks for existing tags and re-writes them to the end. It uses the same method to update the as the inline textarea on MAL (although without the javscript bugs, the server side backslash bug is still a problem). It does NOT handle special characters at this time, but they are rarely used anyways.
It might work on the manga list as well, but hasn't been tested.
All of the input elements have tooltips to let you know what they are for.
Replacements
[ID]
Anime ID
[IMGURL]
Anime cover image URL
[IMGURLT]
Small anime cover image URL
[IMGURLV]
Very small anime cover image URL
[TITLE]
Anime title
[ENGTITLE]
English anime title
[GENRES]
Anime genres comma separated
[THEME]
Anime theme comma separated
[DEMOGRAPHIC]
Anime demographic comma separated
[STUDIOS]
Anime studios comma separated
[PRODUCERS]
Anime producers comma separated
[SEASON]
Season airing started
[YEAR]
Year airing started
[RANK]
Current anime rank
[SCORE]
Current anime score
[STARTDATE]
Airing start date
[ENDDATE]
Airing end date
[DESC]
Anime synopsis
[DEL]
Nothing, it get's deleted (for preventing closing comments "*/" from breaking the header comment in the output)
Code
Modify the templates and other constants at the top to setup your bookmarklet for your what your list needs.
The default is setup for my list. A more common template is: "#more[ID] { background-image: url([IMGURL]); }"
To use, create a bookmark and copy/paste the following script content into the location. When on your list, click the bookmark to activate the tool.
I know it's not pretty, but it gets the job done. Blame MAL's awful markup for this.
Change Log
2022-04-03
Fix for theme and demographic.
2020-10-11
Fix for apostrophe.
2020-04-26
Fix for changes to genres and images.
2018-08-10
Tags AJAX switched to POST with csrf_token and new headers.
2018-05-27.2
Added ability to check if images are valid.
Fixed crash if tags are not shown on list.
Fixed 'undefined' reference error when score is 'N/A'.
Errors are now logged to console and counted to avoid alert popup spam when MAL isn't behaving.
2015-12-21
Added replacements for [STUDIOS], [PRODUCERS], [SEASON], [YEAR] for CSS generation.
2015-12-17
MAL moved studio to it's own category, so a separate option for Studio has been added for tags. Fixed a bug when there is no finished airing date.
2015-12-13
Fixed removal of trailing white space. Added options to include Aired, Score, and Rank in tags.