TYPO3 Core

The attribute is called "accept-charset" (http://www.w3schools.com/tags/att_form_accept_charset.asp). Try it and you'll get an empty page, just tested it 2 minutes ago with a TYPO3 6.2.9.

("accept" is used for mimetypes, see http://www.w3schools.com/tags/att_form_accept.asp)

Works perfectly fine for me in current master.

Tested just a second ago on TYPO3 6.2.15:

prefix = tx_form
confirmation = 0
accept-charset=UTF-8
postProcessor {
    1 = mail
    1 {
        recipientEmail = host@domain.tld
        senderEmail = host@domain.tld
    }
}
10 = SELECT
10 {
    name = standort
    label {
        value = Standort
    }
    10 = OPTION
    10 {
        data = Bitte wählen
value = Keine Auswahl
    }
    20 = OPTION
    20 {
        data = Ort1
value = Ort1
    }
    30 = OPTION
    30 {
        data = Ort2
value = Ort2
    }
    40 = OPTION
    40 {
        data = Ort3
value = Ort3
    }
}
20 = SELECT
20 {
    name = anrede
    label {
        value = Anrede
    }
    10 = OPTION
    10 {
        data = Herr
    }
    20 = OPTION
    20 {
        data = Frau
    }
}
30 = TEXTLINE
30 {
    name = nachname
    label {
        value = Nachname*
    }
}
40 = TEXTLINE
40 {
    name = vorname
    label {
        value = Vorname*
    }
}

90 = TEXTAREA
90 {
    cols = 40
    rows = 5
    name = nachricht
    label {
        value = Ihre Nachricht*
    }
}

110 = SUBMIT
110 {
    name = 5
    value = Abschicken
}
rules {
    1 = required
    1 {
        breakOnError = 0
        showMessage = 1
        message = Benötigt
        error = Dies ist ein Pflichtfeld
        element = nachname
    }
    2 = required
    2 {
        breakOnError = 0
        showMessage = 1
        message = Benötigt
        error = Dies ist ein Pflichtfeld
        element = vorname
    }
    4 = required
    4 {
        breakOnError = 0
        showMessage = 1
        message = Benötigt
        error = Dies ist ein Pflichtfeld
        element = nachricht
    }
}

Result:

[Mon Nov 02 08:52:39 2015] [error] [client xxx.xxx.xxx.xxx] PHP Fatal error: Class 'TYPO3\\CMS\\Form\\Domain\\Model\\Attribute\\Accept-charsetAttribute' not found in /var/www/xxx/html/typo3/sysext/core/Classes/Utility/GeneralUtility.php on line 4440, referer: http://www.domain.tld/

Patch set 2 for branch TYPO3_6-2 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/44955

Patch set 3 for branch TYPO3_6-2 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/44955