А> <Span>                      
А>    <Run Text="aaa" />
А>    <Run Text="bbb" />
А>    <Run Text="ccc" />
А> </Span>
А>

        <InlineUIContainer>
            <Border BorderThickness="1" BorderBrush="Red">
                <TextBlock >
                    <Span>                      
                            <Run Text="aaa" />
                            <Run Text="bbb" />
                            <Run Text="ccc" />
                    </Span>
                </TextBlock>
            </Border>
        </InlineUIContainer>