diff --git a/src/calibre/spell/break_iterator.py b/src/calibre/spell/break_iterator.py
index cba9b3cf8c..65b3828ae2 100644
--- a/src/calibre/spell/break_iterator.py
+++ b/src/calibre/spell/break_iterator.py
@@ -4,7 +4,9 @@
 __license__ = 'GPL v3'
 __copyright__ = '2014, Kovid Goyal <kovid at kovidgoyal.net>'
 
+from collections import defaultdict
 from threading import Lock
+from typing import NamedTuple
 
 from calibre.utils.icu import _icu
 from calibre.utils.localization import lang_as_iso639_1
@@ -125,3 +127,318 @@ def split_into_sentences_for_tts(
             yield start, sentence
     if pending_sentence:
         yield pending_start, pending_sentence
+
+
+class Sentence(NamedTuple):
+    elem_id: str
+    text: str
+    lang: str
+
+
+def mark_sentences_in_html(root, lang: str = '') -> list[Sentence]:
+    from lxml.etree import ElementBase as Element
+    from lxml.etree import tostring as _tostring
+
+    from calibre.ebooks.oeb.base import barename
+    from calibre.utils.localization import canonicalize_lang, get_lang
+    continued_tag_names = frozenset({
+        'a', 'span', 'em', 'strong', 'b', 'i', 'u', 'code', 'sub', 'sup', 'cite', 'q', 'kbd'
+    })
+    ignored_tag_names = frozenset({
+        'img', 'object', 'script', 'style', 'head', 'title', 'form', 'input', 'br', 'hr', 'map', 'textarea', 'svg', 'math', 'rp', 'rt', 'rtc',
+    })
+
+    def tostring(x) -> str:
+        return _tostring(x, encoding='unicode')
+
+    def lang_for_elem(elem, parent_lang):
+        return canonicalize_lang(elem.get('lang') or elem.get('xml_lang') or elem.get('{http://www.w3.org/XML/1998/namespace}lang')) or parent_lang
+
+    def has_text(elem):
+        if elem.text and elem.text.strip():
+            return True
+        for child in elem:
+            if child.tail and child.tail.strip():
+                return True
+        return False
+
+    root_lang = lang_for_elem(root, canonicalize_lang(lang or get_lang()))
+    seen_ids = set(root.xpath('//*/@id'))
+    id_counter = 1
+    ans = []
+    clones_map = defaultdict(list)
+
+    class Chunk(NamedTuple):
+        child: Element | None
+        text: str
+        start_at: int
+        is_tail: bool = False
+
+
+    class Parent:
+
+        def __init__(self, elem, tag_name, parent_lang, child_lang=''):
+            self.elem = elem
+            self.tag_name = tag_name
+            self.lang = child_lang or lang_for_elem(elem, parent_lang)
+            self.pos = 0
+            self.texts = []
+            if elem.text and elem.text.strip():
+                self.texts.append(Chunk(None, elem.text, self.pos))
+                self.pos += len(elem.text)
+            self.children = tuple(elem.iterchildren())
+            self.child_pos = 0
+
+        def add_simple_child(self, elem):
+            if text := elem.text:
+                self.texts.append(Chunk(elem, text, self.pos))
+                self.pos += len(text)
+
+        def add_tail(self, elem, text):
+            self.texts.append(Chunk(elem, text, self.pos, is_tail=True))
+            self.pos += len(text)
+
+        def commit(self) -> None:
+            if not self.texts:
+                return
+            text = ''.join(c.text for c in self.texts)
+            self.pos = 0
+            for start, length in sentence_positions(text, self.lang):
+                elem_id = self.wrap_sentence(start, length)
+                ans.append(Sentence(elem_id, text[start:start+length], lang))
+            self.texts = []
+            self.pos = 0
+
+        def make_into_wrapper(self, elem: Element) -> str:
+            nonlocal id_counter
+            while True:
+                q = f'cttsw-{id_counter}'
+                if q not in seen_ids:
+                    elem.set('id', q)
+                    seen_ids.add(q)
+                    return q
+                id_counter += 1
+
+        def make_wrapper(self, text: str | None) -> Element:
+            ns, sep, _ = self.elem.tag.partition('}')
+            ans = self.elem.makeelement(ns + sep + 'span')
+            ans.text = text
+            self.make_into_wrapper(ans)
+            return ans
+
+        def replace_reference_to_child(self, elem: Element, replacement: Element) -> None:
+            for i in range(self.pos + 1, len(self.texts)):
+                if self.texts[i].child is elem:
+                    self.texts[i] = self.texts[i]._replace(child=replacement)
+                else:
+                    break
+
+        def wrap_contents(self, first_child: Element | None, last_child: Element) -> Element:
+            w = self.make_wrapper(self.elem.text if first_child is None else None)
+            in_range = False
+            for c in self.elem.iterchildren('*'):
+                if not in_range and (first_child is None or first_child is c):
+                    in_range = True
+                    pos = self.elem.index(c)
+                    self.elem.insert(pos, w)
+                    w.append(c)
+                    first_child = c
+                if in_range:
+                    if last_child is not first_child:
+                        w.append(last_child)
+                    if c is last_child:
+                        break
+            self.replace_reference_to_child(last_child, w)
+            return w
+
+        def clone_simple_element(self, elem: Element) -> Element:
+            ans = elem.makeelement(elem.tag)
+            ans.attrib.update(elem.attrib)
+            ans.attrib.pop('id', None)
+            ans.attrib.pop('name', None)
+            ans.text, ans.tail = elem.text, elem.tail
+            p = elem.getparent()
+            idx = p.index(elem)
+            p.insert(idx + 1, ans)
+            self.replace_reference_to_child(elem, ans)
+            clones_map[elem].append(ans)
+            return ans
+
+        def wrap_sentence(self, start: int, length: int) -> str:
+            end = start + length
+            start_chunk = end_chunk = -1
+            start_offset = end_offset = 0
+            for i in range(self.pos, len(self.texts)):
+                c = self.texts[i]
+                if c.start_at <= start:
+                    start_chunk = i
+                    start_offset = start - c.start_at
+                if end <= c.start_at + len(c.text):
+                    end_chunk = i
+                    self.pos = i
+                    end_offset = end - c.start_at
+                    break
+            else:
+                self.pos = end_chunk = len(self.texts) - 1
+                end_offset = len(self.texts[-1].text)
+            assert start_chunk > -1
+            s, e = self.texts[start_chunk], self.texts[end_chunk]
+            if s.child is None: # start in leading text of parent element
+                if e is s:  # end also in leading text of parent element
+                    before, sentence, after = s.text[:start_offset], s.text[start_offset:end_offset], s.text[end_offset:]
+                    self.elem.text = before
+                    w = self.make_wrapper(sentence)
+                    self.elem.insert(0, w)
+                    w.tail = after
+                    if after:
+                        self.texts[self.pos] = Chunk(w, after, end, is_tail=True)
+                    else:
+                        self.pos += 1
+                    return w.get('id')
+                if e.is_tail:  # ending in the tail of a child
+                    before_start, after_start = s.text[:start_offset], s.text[start_offset:]
+                    included, after = e.text[:end_offset], e.text[end_offset:]
+                    e.child.tail = included
+                    self.elem.text = after_start
+                    w = self.wrap_contents(None, e.child)
+                    w.tail = after
+                    self.elem.text = before_start
+                    if after:
+                        self.texts[self.pos] = Chunk(w, after, end, is_tail=True)
+                    else:
+                        self.pos += 1
+                    return w.get('id')
+                # ending inside a child
+                before_start, after_start = s.text[:start_offset], s.text[start_offset:]
+                included, after = e.text[:end_offset], e.text[end_offset:]
+                e.child.text = included
+                c = self.clone_simple_element(e.child)
+                c.text = after
+                e.child.tail = None
+                self.elem.text = after_start
+                w = self.wrap_contents(None, e.child)
+                self.elem.text = before_start
+                if after:
+                    self.texts[self.pos] = Chunk(c, c.text, end)
+                else:
+                    self.pos += 1
+                return w.get('id')
+            # starting in a child text or tail
+            if s.is_tail:
+                if e.is_tail:
+                    if s is e:  # end in tail of same element
+                        before, sentence, after = s.text[:start_offset], s.text[start_offset:end_offset], s.text[end_offset:]
+                        s.child.tail = before
+                        w = self.make_wrapper(sentence)
+                        w.tail = after
+                        idx = self.elem.index(s.child)
+                        self.elem.insert(idx + 1, w)
+                        if after:
+                            self.texts[self.pos] = Chunk(w, after, end, is_tail=True)
+                        else:
+                            self.pos += 1
+                        return w.get('id')
+                    s.child.tail, after_start = s.text[:start_offset], s.text[start_offset:]
+                    e.child.tail, after_end = e.text[:end_offset], e.text[end_offset:]
+                    idx = self.elem.index(s.child)
+                    w = self.wrap_contents(self.elem[idx+1], e.child)
+                    w.text, w.tail = after_start, after_end
+                    if after_end:
+                        self.texts[self.pos] = Chunk(w, after_end, end, is_tail=True)
+                    else:
+                        self.pos += 1
+                    return w.get('id')
+                # end inside some subsequent simple element
+                s.child.tail, after_start = s.text[:start_offset], s.text[start_offset:]
+                e.child.text, after_end = e.text[:end_offset], e.text[end_offset:]
+                c = self.clone_simple_element(e.child)
+                c.text = after_end
+                e.child.tail = None
+                w = self.wrap_contents(self.elem[self.elem.index(s.child) + 1], e.child)
+                w.text = after_start
+                if after_end:
+                    self.texts[self.pos] = Chunk(c, after_end, end)
+                else:
+                    self.pos += 1
+                return w.get('id')
+            # start is in the text of a simple child
+            if s.child is e.child:
+                if e.is_tail:  # ending in tail of element we start in
+                    before_start, after_start = s.text[:start_offset], s.text[start_offset:]
+                    c = self.clone_simple_element(s.child)
+                    s.child.text, s.child.tail = before_start, None
+                    before_end, after_end = e.text[:end_offset], e.text[end_offset:]
+                    c.text, c.tail = after_start, before_end
+                    w = self.wrap_contents(c, c)
+                    w.tail = after_end
+                    if after_end:
+                        self.texts[self.pos] = Chunk(w, after_end, end, is_tail=True)
+                    else:
+                        self.pos += 1
+                    return w.get('id')
+                # start and end in text of element
+                before, sentence, after = s.text[:start_offset], s.text[start_offset:end_offset], s.text[end_offset:]
+                c = self.clone_simple_element(s.child)
+                s.child.text, s.child.tail = before, None
+                c.text, c.tail = sentence, None
+                c2 = self.clone_simple_element(c)
+                c2.text = after
+                self.make_into_wrapper(c)
+                if after:
+                    self.texts[self.pos] = Chunk(c2, after, end)
+                else:
+                    self.pos += 1
+                return c.get('id')
+            # end is in a subsequent simple child or tail of one
+            s.child.text, after_start = s.text[:start_offset], s.text[start_offset:]
+            c = self.clone_simple_element(s.child)
+            c.text, s.child.tail = after_start, None
+            if e.is_tail:
+                e.child.tail, after_end = e.text[:end_offset], e.text[end_offset:]
+                w = self.wrap_contents(c, e.child)
+                w.tail = after_end
+                if after_end:
+                    self.texts[self.pos] = Chunk(w, after_end, end, is_tail=True)
+                else:
+                    self.pos += 1
+                return w.get('id')
+            # end is in text of subsequent simple child
+            e.child.text, after_end = e.text[:end_offset], e.text[end_offset:]
+            c2 = self.clone_simple_element(e.child)
+            c2.text, e.child.tail = after_end, None
+            w = self.wrap_contents(c, e.child)
+            if after_end:
+                self.texts[self.pos] = Chunk(c2, after_end, end)
+            else:
+                self.pos += 1
+            return w.get('id')
+
+    stack_of_parents = [Parent(elem, 'body', root_lang) for elem in root.iterchildren('*') if barename(elem.tag).lower() == 'body']
+    while stack_of_parents:
+        p = stack_of_parents.pop()
+        if len(p.elem) == 1 and not has_text(p.elem):  # wrapper
+            c = p.elem[0]
+            if isinstance(c.tag, str):
+                stack_of_parents.append(Parent(c, barename(c.tag).lower(), p.lang))
+            continue
+        for i in range(p.child_pos, len(p.children)):
+            child = p.children[i]
+            child_lang = lang_for_elem(child, p.lang)
+            child_tag_name = barename(child.tag).lower() if isinstance(child.tag, str) else ''
+            if child_lang == p.lang and child_tag_name in continued_tag_names and len(child) == 0:
+                p.add_simple_child(child)
+            elif child_tag_name not in ignored_tag_names:
+                stack_of_parents.append(Parent(child, child_tag_name, p.lang, child_lang))
+                p.commit()
+                p.child_pos = i + 1
+                stack_of_parents.append(p)
+                continue
+            if text := child.tail:
+                p.add_tail(child, text)
+        p.commit()
+    for src_elem, clones in clones_map.items():
+        for clone in clones + [src_elem]:
+            if not clone.text and not clone.tail and not clone.get('id') and not clone.get('name'):
+                if (p := clone.getparent()) is not None:
+                    p.remove(clone)
+    return ans
diff --git a/src/calibre/utils/icu_test.py b/src/calibre/utils/icu_test.py
index e482dd4ee0..d9b9c25a43 100644
--- a/src/calibre/utils/icu_test.py
+++ b/src/calibre/utils/icu_test.py
@@ -267,6 +267,49 @@ class TestICU(unittest.TestCase):
         }.items():
             self.ae(expected, list(split_into_sentences_for_tts(sentence, max_sentence_length=40)))
 
+    def test_mark_sentences(self):
+        from html5_parser import parse
+        from lxml import html
+
+        from calibre.spell.break_iterator import mark_sentences_in_html
+        for text, expected in reversed({
+            '<p id=1>hello cruel world': '<body><p id="1"><span id="1">hello cruel world</span></p>',
+
+            '<p>hello <b>cruel</b> world': '<body><p><span id="1">hello <b>cruel</b> world</span></p>',
+
+            '<p>Yes, please. Hello <b>cruel</b> world.':
+            '<body><p><span id="1">Yes, please. </span><span id="2">Hello <b>cruel</b> world.</span></p>',
+
+            '<p>Hello <b>cruel</b> <i>world.  </i>':
+            '<body><p><span id="1">Hello <b>cruel</b> <i>world.  </i></span></p>',
+
+            '<p>Yes, <b>please.</b> Well done! Bravissima! ':
+            '<body><p><span id="1">Yes, <b>please.</b> </span><span id="2">Well done! </span><span id="3">Bravissima! </span></p>',
+
+            '<p>Yes, <b>please.</b> Well <i>done! </i>Bravissima! ':
+            '<body><p><span id="1">Yes, <b>please.</b> </span><span id="2">Well <i>done! </i></span><span id="3">Bravissima! </span></p>',
+
+            '<p><i>Hello</i>, world! Good day to you':
+            '<body><p><span id="1"><i>Hello</i>, world! </span><span id="2">Good day to you</span></p>',
+
+            '<p><i>Hello, world! </i>Good day to you':
+            '<body><p><i id="1">Hello, world! </i><span id="2">Good day to you</span></p>',
+
+            '<p><i>Hello, </i><b>world!</b>Good day to you':
+            '<body><p><span id="1"><i>Hello, </i><b>world!</b></span><span id="2">Good day to you</span></p>',
+
+            '<p><i>Hello, </i><b>world</b>! Good day to you':
+            '<body><p><span id="1"><i>Hello, </i><b>world</b>! </span><span id="2">Good day to you</span></p>',
+        }.items()):
+            root = parse(text, namespace_elements=True)
+            mark_sentences_in_html(root)
+            actual = html.tostring(root, encoding='unicode')
+            actual = actual[actual.find('<body'):]
+            actual = actual[:actual.rfind('</body>')]
+            actual = actual.replace('cttsw-', '')
+            self.ae(expected, actual)
+
+
 
 def find_tests():
     return unittest.defaultTestLoader.loadTestsFromTestCase(TestICU)