diff --git a/src/calibre/ebooks/html_transform_rules.py b/src/calibre/ebooks/html_transform_rules.py
new file mode 100644
index 0000000000..351948ed2b
--- /dev/null
+++ b/src/calibre/ebooks/html_transform_rules.py
@@ -0,0 +1,123 @@
+#!/usr/bin/env python
+# vim:fileencoding=utf-8
+# License: GPLv3 Copyright: 2016, Kovid Goyal <kovid at kovidgoyal.net>
+
+
+from calibre.utils.serialize import json_dumps, json_loads
+
+
+class Action:
+
+    def __init__(self, name, short_text, long_text, placeholder=''):
+        self.name = name
+        self.short_text = short_text
+        self.long_text = long_text
+        self.placeholder = placeholder
+
+
+ACTION_MAP = {a.name: a for a in (
+    Action('rename', _('Change tag name'), _('Rename tag to the specified name'), _('New tag name')),
+    Action('remove', _('Remove tag and children'), _('Remove the tag and all its contents')),
+    Action('unwrap', _('Remove tag only'), _('Remove the tag but keep its contents')),
+    Action('add_classes', _('Add classes'), _('Add the specified classes, for e.g.:') + ' bold green', _('Space separated class names')),
+    Action('remove_classes', _('Remove classes'), _('Remove the specified classes, for e.g:') + ' bold green', _('Space separated class names')),
+    Action('wrap', _('Wrap the tag'), _(
+        'Wrap the tag in the specified tag, for example: {0} will wrap the tag in a DIV tag with class {1}').format(
+            '&lt;div class="box"&gt;', 'box'), _('A HTML opening tag')),
+    Action('remove_attrs', _('Remove attributes'), _(
+        'Remove the specified attributes from the tag. Multiple attribute names should be separated by spaces'), _('Space separated attribute names')),
+    Action('add_attrs', _('Add attributes'), _('Add the specified attributes, for e.g.:') + ' class="red" name="test"', _('Space separated attribute names')),
+    Action('empty', _('Empty the tag'), _('Remove all contents from the tag')),
+    Action('insert', _('Insert HTML at start'), _(
+        'The specified HTML snippet is inserted after the opening tag. Note that only valid HTML snippets can be used without unclosed tags'),
+           _('HTML snippet')),
+    Action('insert_end', _('Insert HTML at end'), _(
+        'The specified HTML snippet is inserted before the closing tag. Note that only valid HTML snippets can be used without unclosed tags'),
+           _('HTML snippet')),
+    Action('prepend', _('Insert HTML before tag'), _(
+        'The specified HTML snippet is inserted before the opening tag. Note that only valid HTML snippets can be used without unclosed tags'),
+           _('HTML snippet')),
+    Action('append', _('Insert HTML after tag'), _(
+        'The specified HTML snippet is inserted after the closing tag. Note that only valid HTML snippets can be used without unclosed tags'),
+           _('HTML snippet')),
+)}
+
+
+class Match:
+
+    def __init__(self, name, text, placeholder=''):
+        self.name = name
+        self.text = text
+        self.placeholder = placeholder
+
+
+MATCH_TYPE_MAP = {m.name: m for m in (
+    Match('is', _('is'), _('Tag name')),
+    Match('has_class', _('has class'), _('Class name')),
+    Match('not_has_class', _('does not have class'), _('Class name')),
+    Match('css', _('matches CSS selector'), _('CSS selector')),
+    Match('xpath', _('matches XPath selector'), _('XPath selector')),
+    Match('*', _('is any tag')),
+)}
+
+allowed_keys = frozenset('property match_type query action action_data'.split())
+
+
+def validate_rule(rule):
+    return None, None
+
+
+def compile_rules(serialized_rules):
+    raise NotImplementedError('TODO: Implement this')
+
+
+def transform_container(container, serialized_rules, names=()):
+    rules = compile_rules(serialized_rules)
+    rules
+    raise NotImplementedError('TODO: Implement this')
+
+
+def rule_to_text(rule):
+    def get(prop):
+        return rule.get(prop) or ''
+    text = _('If the tag {match_type} {query}').format(
+        match_type=MATCH_TYPE_MAP[rule['match_type']].short_text, query=get('query'))
+    for action in rule['actions']:
+        text += '\n'
+        text += _('{action_type} {action_data}').format(
+            action_type=ACTION_MAP[action['type']].short_text, action_data=action['data'])
+    return text
+
+
+def export_rules(serialized_rules):
+    return json_dumps(serialized_rules, indent=2, sort_keys=True)
+
+
+def import_rules(raw_data):
+    return json_loads(raw_data)
+
+
+def test(return_tests=False):  # {{{
+    import unittest
+
+    class TestTransforms(unittest.TestCase):
+        longMessage = True
+        maxDiff = None
+        ae = unittest.TestCase.assertEqual
+
+        def test_matching(self):
+            pass
+
+        def test_export_import(self):
+            rule = {'property':'a', 'match_type':'*', 'query':'some text', 'action':'remove', 'action_data':'color: red; a: b'}
+            self.ae(rule, next(import_rules(export_rules([rule]))))
+
+    tests = unittest.defaultTestLoader.loadTestsFromTestCase(TestTransforms)
+    if return_tests:
+        return tests
+    unittest.TextTestRunner(verbosity=4).run(tests)
+
+
+if __name__ == '__main__':
+    test()
+# }}}
diff --git a/src/calibre/gui2/html_transform_rules.py b/src/calibre/gui2/html_transform_rules.py
new file mode 100644
index 0000000000..a6cb87734f
--- /dev/null
+++ b/src/calibre/gui2/html_transform_rules.py
@@ -0,0 +1,430 @@
+#!/usr/bin/env python
+# vim:fileencoding=utf-8
+# License: GPLv3 Copyright: 2016, Kovid Goyal <kovid at kovidgoyal.net>
+
+
+from qt.core import (
+    QComboBox, QDialogButtonBox, QFrame, QHBoxLayout, QIcon, QLabel, QLineEdit,
+    QMenu, QPushButton, QScrollArea, QSize, Qt, QTextCursor, QToolButton,
+    QVBoxLayout, QWidget, pyqtSignal
+)
+
+from calibre.ebooks.html_transform_rules import (
+    ACTION_MAP, MATCH_TYPE_MAP, compile_rules, export_rules, import_rules,
+    validate_rule
+)
+from calibre.gui2 import choose_files, choose_save_file, elided_text, error_dialog
+from calibre.gui2.tag_mapper import (
+    RuleEditDialog as RuleEditDialogBase, RuleItem as RuleItemBase,
+    Rules as RulesBase, RulesDialog as RulesDialogBase, SaveLoadMixin
+)
+from calibre.gui2.widgets2 import Dialog
+from calibre.utils.config import JSONConfig
+from calibre.utils.localization import localize_user_manual_link
+
+
+class TagAction(QWidget):
+
+    remove_action = pyqtSignal(object)
+
+    def __init__(self, parent=None):
+        super().__init__(parent)
+        self.l = l = QVBoxLayout(self)
+        self.h = h = QHBoxLayout()
+        l.addLayout(h)
+
+        english_sentence = '{action_type} {action_data}'
+        sentence = _('{action_type} {action_data}')
+        if set(sentence.split()) != set(english_sentence.split()):
+            sentence = english_sentence
+        parts = sentence.split()
+        for clause in parts:
+            if clause == '{action_data}':
+                self.action_data = w = QLineEdit(self)
+                w.setClearButtonEnabled(True)
+            elif clause == '{action_type}':
+                self.action_type = w = QComboBox(self)
+                for action, ac in ACTION_MAP.items():
+                    w.addItem(ac.short_text, action)
+                w.currentIndexChanged.connect(self.update_state)
+            h.addWidget(w)
+            if clause is not parts[-1]:
+                h.addWidget(QLabel('\xa0'))
+        self.h2 = h = QHBoxLayout()
+        l.addLayout(h)
+
+        self.remove_button = b = QToolButton(self)
+        b.setToolTip(_('Remove this action')), b.setIcon(QIcon(I('minus.png')))
+        b.clicked.connect(self.request_remove)
+        h.addWidget(b)
+        self.action_desc = la = QLabel('')
+        la.setWordWrap(True)
+        la.setTextFormat(Qt.TextFormat.RichText)
+        h.addWidget(la)
+        self.sep = sep = QFrame(self)
+        sep.setFrameShape(QFrame.Shape.HLine)
+        l.addWidget(sep)
+        self.update_state()
+
+    def request_remove(self):
+        self.remove_action.emit(self)
+
+    @property
+    def as_dict(self):
+        return {'type': self.action_type.currentData(), 'data': self.action_data.text()}
+
+    @as_dict.setter
+    def as_dict(self, val):
+        self.action_data.setText(val.get('data') or '')
+        at = val.get('type')
+        if at:
+            idx = self.action_type.findData(at)
+            if idx > -1:
+                self.action_type.setCurrentIndex(idx)
+
+    def update_state(self):
+        val = self.as_dict
+        ac = ACTION_MAP[val['type']]
+        self.action_desc.setText(ac.long_text)
+        if ac.placeholder:
+            self.action_data.setVisible(True)
+            self.action_data.setPlaceholderText(ac.placeholder)
+        else:
+            self.action_data.setVisible(False)
+
+
+class ActionsContainer(QScrollArea):
+
+    def __init__(self, parent=None):
+        super().__init__(parent)
+        self.setWidgetResizable(True)
+        self.w = w = QWidget()
+        self.setWidget(w)
+        w.l = QVBoxLayout(w)
+        w.l.addStretch(1)
+        self.all_actions = []
+        self.new_action()
+
+    def new_action(self):
+        a = TagAction(self)
+        self.all_actions.append(a)
+        l = self.w.l
+        a.remove_action.connect(self.remove_action)
+        l.insertWidget(l.count() - 1, a)
+        a.action_type.setFocus(Qt.FocusReason.OtherFocusReason)
+        return a
+
+    def remove_action(self, ac):
+        if ac in self.all_actions:
+            self.w.l.removeWidget(ac)
+            del self.all_actions[self.all_actions.index(ac)]
+            ac.deleteLater()
+
+    def sizeHint(self):
+        ans = super().sizeHint()
+        ans.setHeight(ans.height() + 200)
+        return ans
+
+    @property
+    def as_list(self):
+        return [t.as_dict for t in self.all_actions]
+
+    @as_list.setter
+    def as_list(self, val):
+        for ac in tuple(self.all_actions):
+            self.remove_action(ac)
+        for entry in val:
+            self.new_action().as_dict = entry
+
+
+class RuleEdit(QWidget):  # {{{
+
+    MSG = _('Create the rule to transform HTML tags below')
+
+    def __init__(self, parent=None):
+        QWidget.__init__(self, parent)
+        self.l = l = QVBoxLayout(self)
+        self.h = h = QHBoxLayout()
+
+        self.la = la = QLabel(self.MSG)
+        la.setWordWrap(True)
+        l.addWidget(la)
+        l.addLayout(h)
+        english_sentence = '{preamble} {match_type}'
+        sentence = _('{preamble} {match_type} {query}')
+        if set(sentence.split()) != set(english_sentence.split()):
+            sentence = english_sentence
+        parts = sentence.split()
+        for clause in parts:
+            if clause == '{preamble}':
+                self.preamble = w = QLabel(_('If the tag'))
+            elif clause == '{match_type}':
+                self.match_type = w = QComboBox(self)
+                for action, m in MATCH_TYPE_MAP.items():
+                    w.addItem(m.text, action)
+                w.currentIndexChanged.connect(self.update_state)
+            h.addWidget(w)
+            if clause is not parts[-1]:
+                h.addWidget(QLabel('\xa0'))
+        h.addStretch(1)
+        self.hl = h = QHBoxLayout()
+        l.addLayout(h)
+        self.query = q = QLineEdit(self)
+        q.setClearButtonEnabled(True)
+        h.addWidget(q)
+        h.addSpacing(20)
+        self.query_help_label = la = QLabel(self)
+        la.setOpenExternalLinks(True)
+        h.addWidget(la)
+
+        self.thenl = QLabel(_('Then:'))
+        l.addWidget(self.thenl)
+        self.actions = a = ActionsContainer(self)
+        l.addWidget(a)
+        self.add_button = b = QPushButton(QIcon(I('plus.png')), _('Add another action'))
+        b.clicked.connect(self.actions.new_action)
+        l.addWidget(b)
+        self.update_state()
+
+    def sizeHint(self):
+        a = QWidget.sizeHint(self)
+        a.setHeight(a.height() + 375)
+        a.setWidth(a.width() + 125)
+        return a
+
+    def update_state(self):
+        r = self.rule
+        mt = r['match_type']
+        self.query.setVisible(mt != '*')
+        self.query.setPlaceholderText(MATCH_TYPE_MAP[mt].placeholder)
+        self.query_help_label.setVisible(mt in ('css', 'xpath'))
+        if self.query_help_label.isVisible():
+            if mt == 'css':
+                url = 'https://developer.mozilla.org/en-US/docs/Learn/CSS/Building_blocks/Selectors'
+                text = _('CSS Selector help')
+            else:
+                url = localize_user_manual_link('https://manual.calibre-ebook.com/xpath.html')
+                text = _('XPath selector help')
+            self.query_help_label.setText(f'<a href="{url}">{text}</a>')
+
+    @property
+    def rule(self):
+        return {
+            'match_type': self.match_type.currentData(),
+            'query': self.query.text().strip(),
+            'actions': self.actions.as_list,
+        }
+
+    @rule.setter
+    def rule(self, rule):
+        def sc(name):
+            c = getattr(self, name)
+            c.setCurrentIndex(max(0, c.findData(str(rule.get(name, '')))))
+        sc('match_type')
+        self.query.setText(str(rule.get('query', '')).strip())
+        self.actions.as_list = rule.get('actions') or []
+        self.update_state()
+
+    def validate(self):
+        rule = self.rule
+        title, msg = validate_rule(rule)
+        if msg is not None and title is not None:
+            error_dialog(self, title, msg, show=True)
+            return False
+        return True
+# }}}
+
+
+class RuleEditDialog(RuleEditDialogBase):  # {{{
+
+    PREFS_NAME = 'edit-css-transform-rule'
+    DIALOG_TITLE = _('Edit rule')
+    RuleEditClass = RuleEdit
+# }}}
+
+
+class RuleItem(RuleItemBase):  # {{{
+
+    @staticmethod
+    def text_from_rule(rule, parent):
+        try:
+            query = elided_text(rule['query'], font=parent.font(), width=200, pos='right')
+            text = _(
+                'If the property <i>{property}</i> <b>{match_type}</b> <b>{query}</b><br>{action}').format(
+                    property=rule['property'], action=ACTION_MAP[rule['action']],
+                    match_type=MATCH_TYPE_MAP[rule['match_type']].text, query=query)
+            if rule['action_data']:
+                ad = elided_text(rule['action_data'], font=parent.font(), width=200, pos='right')
+                text += ' <code>%s</code>' % ad
+        except Exception:
+            import traceback
+            traceback.print_exc()
+            text = _('This rule is invalid, please remove it')
+        return text
+# }}}
+
+
+class Rules(RulesBase):  # {{{
+
+    RuleItemClass = RuleItem
+    RuleEditDialogClass = RuleEditDialog
+
+    MSG = _('You can specify rules to transform styles here. Click the "Add rule" button'
+            ' below to get started.')
+# }}}
+
+
+class Tester(Dialog):  # {{{
+
+    DIALOG_TITLE = _('Test style transform rules')
+    PREFS_NAME = 'test-style-transform-rules'
+    LABEL = _('Enter a CSS stylesheet below to test')
+
+    def __init__(self, rules, parent=None):
+        self.rules = compile_rules(rules)
+        Dialog.__init__(self, self.DIALOG_TITLE, self.PREFS_NAME, parent=parent)
+
+    def setup_ui(self):
+        from calibre.gui2.tweak_book.editor.text import TextEdit
+        self.l = l = QVBoxLayout(self)
+        self.bb.setStandardButtons(QDialogButtonBox.StandardButton.Close)
+        self.la = la = QLabel(self.LABEL)
+        l.addWidget(la)
+        self.css = t = TextEdit(self)
+        t.load_text('/* %s */\n' % _('Enter CSS rules below and click the "Test" button'), 'css')
+        la.setBuddy(t)
+        c = t.textCursor()
+        c.movePosition(QTextCursor.MoveOperation.End)
+        t.setTextCursor(c)
+        self.h = h = QHBoxLayout()
+        l.addLayout(h)
+        h.addWidget(t)
+        self.test_button = b = QPushButton(_('&Test'), self)
+        b.clicked.connect(self.do_test)
+        h.addWidget(b)
+        self.result = la = TextEdit(self)
+        la.setReadOnly(True)
+        l.addWidget(la)
+        l.addWidget(self.bb)
+
+    @property
+    def value(self):
+        return self.css.toPlainText()
+
+    def do_test(self):
+        raise NotImplementedError('TODO: Implement this')
+
+    def sizeHint(self):
+        return QSize(800, 600)
+# }}}
+
+
+class RulesDialog(RulesDialogBase):  # {{{
+
+    DIALOG_TITLE = _('Edit style transform rules')
+    PREFS_NAME = 'edit-style-transform-rules'
+    RulesClass = Rules
+    TesterClass = Tester
+
+    def __init__(self, *args, **kw):
+        # This has to be loaded on instantiation as it can be shared by
+        # multiple processes
+        self.PREFS_OBJECT = JSONConfig('style-transform-rules')
+        RulesDialogBase.__init__(self, *args, **kw)
+# }}}
+
+
+class RulesWidget(QWidget, SaveLoadMixin):  # {{{
+
+    changed = pyqtSignal()
+
+    def __init__(self, parent=None):
+        self.loaded_ruleset = None
+        QWidget.__init__(self, parent)
+        self.PREFS_OBJECT = JSONConfig('style-transform-rules')
+        l = QVBoxLayout(self)
+        self.rules_widget = w = Rules(self)
+        w.changed.connect(self.changed.emit)
+        l.addWidget(w)
+        self.h = h = QHBoxLayout()
+        l.addLayout(h)
+        self.export_button = b = QPushButton(_('E&xport'), self)
+        b.setToolTip(_('Export these rules to a file'))
+        b.clicked.connect(self.export_rules)
+        h.addWidget(b)
+        self.import_button = b = QPushButton(_('&Import'), self)
+        b.setToolTip(_('Import previously exported rules'))
+        b.clicked.connect(self.import_rules)
+        h.addWidget(b)
+        self.test_button = b = QPushButton(_('&Test rules'), self)
+        b.clicked.connect(self.test_rules)
+        h.addWidget(b)
+        h.addStretch(10)
+        self.save_button = b = QPushButton(_('&Save'), self)
+        b.setToolTip(_('Save this ruleset for later re-use'))
+        b.clicked.connect(self.save_ruleset)
+        h.addWidget(b)
+        self.export_button = b = QPushButton(_('&Load'), self)
+        self.load_menu = QMenu(self)
+        b.setMenu(self.load_menu)
+        b.setToolTip(_('Load a previously saved ruleset'))
+        b.clicked.connect(self.load_ruleset)
+        h.addWidget(b)
+        self.build_load_menu()
+
+    def export_rules(self):
+        rules = self.rules_widget.rules
+        if not rules:
+            return error_dialog(self, _('No rules'), _(
+                'There are no rules to export'), show=True)
+        path = choose_save_file(self, 'export-style-transform-rules', _('Choose file for exported rules'), initial_filename='rules.txt')
+        if path:
+            raw = export_rules(rules)
+            with open(path, 'wb') as f:
+                f.write(raw)
+
+    def import_rules(self):
+        paths = choose_files(self, 'export-style-transform-rules', _('Choose file to import rules from'), select_only_single_file=True)
+        if paths:
+            with open(paths[0], 'rb') as f:
+                rules = import_rules(f.read())
+            self.rules_widget.rules = list(rules) + list(self.rules_widget.rules)
+            self.changed.emit()
+
+    def load_ruleset(self, name):
+        SaveLoadMixin.load_ruleset(self, name)
+        self.changed.emit()
+
+    def test_rules(self):
+        Tester(self.rules_widget.rules, self).exec_()
+
+    @property
+    def rules(self):
+        return self.rules_widget.rules
+
+    @rules.setter
+    def rules(self, val):
+        try:
+            self.rules_widget.rules = val or []
+        except Exception:
+            import traceback
+            traceback.print_exc()
+            self.rules_widget.rules = []
+# }}}
+
+
+if __name__ == '__main__':
+    from calibre.gui2 import Application
+    app = Application([])
+    v = RuleEdit()
+    v.setWindowFlag(Qt.WindowType.Dialog)
+    v.show()
+    app.exec_()
+    # d = RulesDialog()
+    # d.rules = [
+    #     {'match_type':'*', 'query':'', 'action':'change', 'action_data':'green'},
+    # ]
+    # d.exec_()
+    # from pprint import pprint
+    # pprint(d.rules)
+    # del d, app