I'm frequently getting unstyled pages or pages which take many seconds to finish loading on the English Wikipedia and other wikis in the last few days. Some requests just fail:
The load.php URL which gave a 503 in the example is (note it reflects some personal preferences, for instance the en-gb).
https://en.wikipedia.org/w/load.php?debug=false&lang=en-gb&modules=ext.3d%2CeventLogging%2CnavigationTiming%2Cpopups%2CwikimediaEvents%7Cext.centralNotice.choiceData%2Cdisplay%2CgeoIP%2CimpressionDiet%2CkvStore%2CkvStoreMaintenance%2CstartUp%7Cext.centralauth.ForeignApi%7Cext.centralauth.centralautologin.clearcookie%7Cext.cite.a11y%7Cext.echo.api%2Cinit%7Cext.eventLogging.subscriber%7Cext.popups.images%7Cext.uls.common%2Ccompactlinks%2Ceventlogger%2Cinit%2Cinterface%2Cpreferences%2Cwebfonts%7Cext.visualEditor.desktopArticleTarget.init%7Cext.visualEditor.supportCheck%2CtargetLoader%2CtempWikitextEditorWidget%2Ctrack%2Cve%7Cext.wikimediaEvents.loggedin%7Cjquery.accessKeyLabel%2CbyteLength%2CcheckboxShiftClick%2Cclient%2Ccookie%2CgetAttrs%2Chidpi%2ChighlightText%2Cmw-jump%2Cspinner%2Csuggestions%2CtextSelection%7Cjquery.uls.data%7Cmediawiki.ForeignApi%2CRegExp%2CString%2CTitle%2CUri%2Capi%2Ccldr%2Ccookie%2Cexperiments%2CjqueryMsg%2Clanguage%2Cnotify%2CsearchSuggest%2Cstorage%2Ctemplate%2Cuser%2Cutil%7Cmediawiki.ForeignApi.core%7Cmediawiki.api.options%2Cuser%2Cwatch%7Cmediawiki.editfont.styles%7Cmediawiki.language.data%2Cinit%7Cmediawiki.libs.pluralruleparser%7Cmediawiki.page.ready%2Cstartup%7Cmediawiki.page.watch.ajax%7Cmediawiki.template.mustache%2Cregexp%7Cmediawiki.ui.button%2Cicon%7Cmmv.bootstrap%2Chead%7Cmmv.bootstrap.autostart%7Coojs%2Csite%7Cschema.UniversalLanguageSelector%7Cuser.defaults%7Cwikibase.client.linkitem.init&skin=monobook&version=1kmh2ku
I did not see obvious network/ Traffic issues such as packet loss. The issue also doesn't seem to be confined to CSS as I thought until yesterday (T181877 is about that).